Boat Plans For A Chesapeake Deadrise
Gday all,
Hope youve had great week as we have.
"World on Water" Latest Global News
Over the past week, you may have noticed an addition that weve added to our Home Page, "World on Water" latest weekly news from around the world. This segment is updated each Friday on our Home Page. Hope you enjoy it!
Tinnie and Tackle Show - Brisbane (08 to 10th Apr)
This week from Friday to Sunday is the annual Boating show. As we plan on going down, well be there on Friday to have a look around at all teh latest toys and boats on show.
We also organising to catch up with a few of our clients whilst at the show, so if your going on Friday, let us know so we can organise a time and spot, as I know that wed all love to meet you (in person) and you can meet our other builders as well and catch up with their happenings!
Contact Us
Slims the Ticket
Take a spoon, any spoon, and hold it vertically by the handle so that it is suspended just outside the water flowing from a running tap - the rounded side into the flow. As you slowly move the spoon towards the water, you’ll notice that the spoon is ‘sucked’ into the flow. If the spoon represents the bottom of a short, fat ‘displacement’ hull running through water it is not hard to imagine that that same ‘suction’ effect draws the boat downwards into the water. The faster the flow, or speed, the greater the suction. This very quickly shows why displacement hulls cannot go faster than hull speed as the power requirements to climb over its own bow wave and also to break free of its suction is so large that it makes the exercise not economical.
This silly-simple exercise demonstrates one small component of the very complex catalogue of the drag forces that combine to resist the forward motion of a hull passing through the water.
Resistance…
The resistance of a boat can be split into three main components - frictional, residuary and air resistance. The relative importance of these components changes as the boat length, displacement and speed increase. For now we can ignore air resistance as it is relatively small.
Frictional resistance is associated with the viscous drag of the underwater surface of the boat and therefore the size of that wetted surface is important, along with its roughness
Residuary resistance is mostly associated with the energy lost by the boat in the generation of waves – the wave-making drag. The waves generated by the boat will have a speed equal to the boat speed.
A full displacement hull is restrained by wave-making drag so that its speed is absolutely limited to a speed which is function of its waterline length. This function is called Speed/Length ratio and turns out to be about 1.3 x the sq. root of the waterline length in ft.
The most common way to escape this natural speed limit is to design the hull so that it is flat enough underneath to cause it to ‘plane’ on the surface of the water. By ‘planing’ (‘skimming would be another word) it is no longer floating and therefore no longer held captive by wave-making drag. Also by keeping her bows sharp to reduce bow build up pressure with a moderate transom deadrise to prevent squatting, we can now push past the S/L ratio of 1.34 and now enter the semi displacement range between 1.4 to 2.5.
For reasons explained above, we have chosen to explore and further develop a different way to reduce wave-making drag – a way that intentionally uses very little of the ‘dynamic’ lift available through planing.
• As the drag of non-planing hulls is governed by a formula in which the expression ‘sq. root of the waterline length’ is dominant, so that simply increasing that length also increases speed
• For a similar displacement (or weight) the maximum cross-section of the hull reduces as the length over which it is spread increases. It comes as no surprise that pushing a smaller cross-section through the water uses less energy than pushing a bigger one.
Waves and Speed…
There is a fundamental law governing waves in deep water - the speed of a wave is proportional to the square root of its wavelength. In other words the faster a boat moves the further apart are the waves generated.?
Consider a boat moving at a speed such that it has wave crests at the bow and the stern then the wavelength is about equal to the waterline length of the boat. This speed is known as the displacement speed and is the maximum speed at which a boat can travel whilst not climbing out of its own wave system. Therefore there is a fixed relationship between the displacement speed and the waterline length of a boat. A boat will have a higher displacement speed simply by being longer.
As an example, in the picture opposite you can see the drawn out waterline in the forward part of the SportzMaster 19. This reduces the build up of pressure at the bow enabling the SM19 to "slice" through her own bow wave whilst getting up on the plane. It also means that while at "trolling" speeds, less HP is required.
?
?
Opposite is a typical hull form found in most alloy boats. Note the forward waterline sections how they appear short and not drawn out. You can quickly see how the pressure builds up and the bow as shown and then moves aft producing a large wave system resulting in drag. This also results in an excessive "bow up" attitude whilst on the plane (more on this in coming articles)
A ‘Low Displacement/Length Ratio’ hull simply has its mass spread out over a suitably long waterline length. The result is a vessel that delivers a quiet and comfortable ride across a wide range of cruising speeds.
When discussing different hydrodynamic phenomena it is worth emphasizing that the boundaries between design characteristics are not demarked by any sudden changes. As a hull is ‘drawn out’, for example, the drag we are trying to reduce (wave-making drag) is reduced progressively as the Displacement/Length ratio is reduced.
The Displacement/Length ratio is determined by the following formula:
DL ratio = Displ. / (0.01 x WL)^3
Where:
Displ. is the displacement in long tons
(2240 lbs)
WL is the waterline length in feet.
A vessel whose DL ratio is below 100 is likely to benefit from some significant decline in wave-making drag. Below that the results just get progressively better.
Speed and Length…
It is important to note that boat speeds can only properly be compared in terms of speed/length ratio or more formally Froude number. If this quantity is the same for boats of different sizes then they are operating in the same kind of wave system and can be compared. For example a 40ft boat moving at 9 knots will be operating in the same wave system as an 80ft boat moving at about 13 knots.
In order to break out of its wave system and exceed its displacement speed a boat will need a suitable hull shape and sufficient propulsive power. The hull will need to be light and designed to generate lift as speed increases. This dynamic lift progressively supports more of the weight of the hull as speed increases, thus diminishing the support provided by the buoyancy of the hull and releasing the hull from its speed limiting wave system.
A hull capable of speeds where boat weight is partly supported by the displacement of the hull and partly by dynamic lift is referred to as a semi-displacement hull. And a hull capable of speeds where boat weight is mostly supported by dynamic lift is called a planing design.
For the purposes of this article, it is important to note that direct comparisons of D/L ratio of are only really useful when considering boats of roughly similar displacement.
In our work to date, the best application for Low Displacement / Length technology appears to be in vessels that are required to travel at ‘medium speed’, which in practice could lie between SL ratios of around 1.3 up to around 3. As and example, the Black Label 30 was designed to operate with max fuel efficiency and comfort in this speed range prior to getting up on the plane and also whilst on the plane at a cruising speed of 27 kts at 3/4 throttle.
The Speed/Length ratio is determined by the following formula:
SL ratio = V / (sq.rt)WL
Where:
V is the boat speed in knots
WL is the waterline length in feet.
By “drawing out” the design, keeping the bows or deadrise at the cutwater sharp and having a moderate deadrise at the transom to prevent squatting, this enables the vessels to be easily driven at low speeds, “slicing” through her own bow pressure wave whilst getting up on the plane, thereby reducing the “bow up” attitude that most short fat power boats have and a lower trim angle whilst on the plane. Overall, this provides a more comfortable ride, reducing HP requirements and increasing fuel efficiency.
With small boats, unfortunatly "slimming" the boat is unavoidable due to stability reasons, however we can improve its performance by drawing out its forward waterline. However, once we begin to get into larger boats..SLIMS THE TICKET.
Latest Build Pictures for the Week
Grahams SportzMaster 19
Hi,
All technical this morning finished off windlass ready for final glassing,positioned scuppers,roughed out fuel tank and completed Furuno in hull transducer at 14 degrees. So believe it or not, it is exactly per plan. Mark is a clever devil !!!.
Cheers
Graham,
You can see more of Grahams SportzMaster 19 in our BoatBuilding Forum
For more information about the the SportzMaster 19 or any other of our range of designs,
please visit our website
Gregs Cruise Control 5.5
Greg popped in for a visit today, stayed for around 4 hours yaking about various changes that he wanted to do to his Cruise Control.
As some of you may not know, Greg initally asked about "stretching" the Cruise Control from 5.2 m to 5.5 metres. So, after a bit of number crunching and cross checking, he got the go ahead.
It was great catching up with him again
Anyway, heres his latest pictures from the beginning, to now
You can see more of Gregs Cruise Control 5.5 in our BoatBuilding Forum
For more information about the the Cruise Control 5.2 or any other of our range of designs,
please visit our website
----------------------------------------
If you have any comments youd like to make, please click the "COMMENTS" link below
See you next week with our latest weekly news and other articles !
Thanks for visiting our website?????????????
http://www.bowdidgemarinedesigns.com/
Hope youve had great week as we have.
"World on Water" Latest Global News
Tinnie and Tackle Show - Brisbane (08 to 10th Apr)
We also organising to catch up with a few of our clients whilst at the show, so if your going on Friday, let us know so we can organise a time and spot, as I know that wed all love to meet you (in person) and you can meet our other builders as well and catch up with their happenings!
Contact Us
Slims the Ticket
This silly-simple exercise demonstrates one small component of the very complex catalogue of the drag forces that combine to resist the forward motion of a hull passing through the water.
Resistance…
The resistance of a boat can be split into three main components - frictional, residuary and air resistance. The relative importance of these components changes as the boat length, displacement and speed increase. For now we can ignore air resistance as it is relatively small.
Frictional resistance is associated with the viscous drag of the underwater surface of the boat and therefore the size of that wetted surface is important, along with its roughness
Residuary resistance is mostly associated with the energy lost by the boat in the generation of waves – the wave-making drag. The waves generated by the boat will have a speed equal to the boat speed.
A full displacement hull is restrained by wave-making drag so that its speed is absolutely limited to a speed which is function of its waterline length. This function is called Speed/Length ratio and turns out to be about 1.3 x the sq. root of the waterline length in ft.
The most common way to escape this natural speed limit is to design the hull so that it is flat enough underneath to cause it to ‘plane’ on the surface of the water. By ‘planing’ (‘skimming would be another word) it is no longer floating and therefore no longer held captive by wave-making drag. Also by keeping her bows sharp to reduce bow build up pressure with a moderate transom deadrise to prevent squatting, we can now push past the S/L ratio of 1.34 and now enter the semi displacement range between 1.4 to 2.5.
For reasons explained above, we have chosen to explore and further develop a different way to reduce wave-making drag – a way that intentionally uses very little of the ‘dynamic’ lift available through planing.
 |
| The SportzMaster 19 |
Instead it involves ‘drawing out’ the hull lines of that short, fat displacement hull so that the length-wise curvature of the hull is much less pronounced, and therefore much less inclined to attract wave-making drag.
Apart from getting rid of some of the curvature, extending the hull offers other drag-reducing benefits at low speed:
• As the drag of non-planing hulls is governed by a formula in which the expression ‘sq. root of the waterline length’ is dominant, so that simply increasing that length also increases speed
• For a similar displacement (or weight) the maximum cross-section of the hull reduces as the length over which it is spread increases. It comes as no surprise that pushing a smaller cross-section through the water uses less energy than pushing a bigger one.
Waves and Speed…
There is a fundamental law governing waves in deep water - the speed of a wave is proportional to the square root of its wavelength. In other words the faster a boat moves the further apart are the waves generated.?
Consider a boat moving at a speed such that it has wave crests at the bow and the stern then the wavelength is about equal to the waterline length of the boat. This speed is known as the displacement speed and is the maximum speed at which a boat can travel whilst not climbing out of its own wave system. Therefore there is a fixed relationship between the displacement speed and the waterline length of a boat. A boat will have a higher displacement speed simply by being longer.
 |
| SportzMaster 19s wave system at 3 kts trolling speed |
As an example, in the picture opposite you can see the drawn out waterline in the forward part of the SportzMaster 19. This reduces the build up of pressure at the bow enabling the SM19 to "slice" through her own bow wave whilst getting up on the plane. It also means that while at "trolling" speeds, less HP is required.
?
?
 |
| A typical alloy boats wave system at 3 kts trolling speed |
Opposite is a typical hull form found in most alloy boats. Note the forward waterline sections how they appear short and not drawn out. You can quickly see how the pressure builds up and the bow as shown and then moves aft producing a large wave system resulting in drag. This also results in an excessive "bow up" attitude whilst on the plane (more on this in coming articles)
A ‘Low Displacement/Length Ratio’ hull simply has its mass spread out over a suitably long waterline length. The result is a vessel that delivers a quiet and comfortable ride across a wide range of cruising speeds.
When discussing different hydrodynamic phenomena it is worth emphasizing that the boundaries between design characteristics are not demarked by any sudden changes. As a hull is ‘drawn out’, for example, the drag we are trying to reduce (wave-making drag) is reduced progressively as the Displacement/Length ratio is reduced.
The Displacement/Length ratio is determined by the following formula:
DL ratio = Displ. / (0.01 x WL)^3
Where:
Displ. is the displacement in long tons
(2240 lbs)
WL is the waterline length in feet.
A vessel whose DL ratio is below 100 is likely to benefit from some significant decline in wave-making drag. Below that the results just get progressively better.
Speed and Length…
It is important to note that boat speeds can only properly be compared in terms of speed/length ratio or more formally Froude number. If this quantity is the same for boats of different sizes then they are operating in the same kind of wave system and can be compared. For example a 40ft boat moving at 9 knots will be operating in the same wave system as an 80ft boat moving at about 13 knots.
In order to break out of its wave system and exceed its displacement speed a boat will need a suitable hull shape and sufficient propulsive power. The hull will need to be light and designed to generate lift as speed increases. This dynamic lift progressively supports more of the weight of the hull as speed increases, thus diminishing the support provided by the buoyancy of the hull and releasing the hull from its speed limiting wave system.
A hull capable of speeds where boat weight is partly supported by the displacement of the hull and partly by dynamic lift is referred to as a semi-displacement hull. And a hull capable of speeds where boat weight is mostly supported by dynamic lift is called a planing design.
For the purposes of this article, it is important to note that direct comparisons of D/L ratio of are only really useful when considering boats of roughly similar displacement.
The Speed/Length ratio is determined by the following formula:
SL ratio = V / (sq.rt)WL
Where:
V is the boat speed in knots
WL is the waterline length in feet.
By “drawing out” the design, keeping the bows or deadrise at the cutwater sharp and having a moderate deadrise at the transom to prevent squatting, this enables the vessels to be easily driven at low speeds, “slicing” through her own bow pressure wave whilst getting up on the plane, thereby reducing the “bow up” attitude that most short fat power boats have and a lower trim angle whilst on the plane. Overall, this provides a more comfortable ride, reducing HP requirements and increasing fuel efficiency.
With small boats, unfortunatly "slimming" the boat is unavoidable due to stability reasons, however we can improve its performance by drawing out its forward waterline. However, once we begin to get into larger boats..SLIMS THE TICKET.
Latest Build Pictures for the Week
Grahams SportzMaster 19
Hi,
Cheers
Graham,
You can see more of Grahams SportzMaster 19 in our BoatBuilding Forum
For more information about the the SportzMaster 19 or any other of our range of designs,
please visit our website
Gregs Cruise Control 5.5
As some of you may not know, Greg initally asked about "stretching" the Cruise Control from 5.2 m to 5.5 metres. So, after a bit of number crunching and cross checking, he got the go ahead.
It was great catching up with him again
Anyway, heres his latest pictures from the beginning, to now
You can see more of Gregs Cruise Control 5.5 in our BoatBuilding Forum
For more information about the the Cruise Control 5.2 or any other of our range of designs,
please visit our website
----------------------------------------
If you have any comments youd like to make, please click the "COMMENTS" link below
See you next week with our latest weekly news and other articles !
Thanks for visiting our website?????????????
http://www.bowdidgemarinedesigns.com/
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