I had a question regarding the boat speed. I was reading the study plans and it said that she is designed to operate around the theoretical hull speed of 7kn and that would require less than 20HP engine. The study plans go on to say that she was designed with a 40/45 HP diesel engine.
So, if I put a 45 HP engine in the boat, how much faster would she go? Would that be putting too much stress on the hull if I was constantly running her at >7kn?
28' Trawler (TW28) build boat speed
28' Trawler (TW28) build boat speed
Check out my Trawler 28 build viewtopic.php?t=66261.
- Corto Maltese
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Re: 28' Trawler (TW28) build boat speed
Maximum speeds for displacement hulls are determined by this formula: the square root of the waterline (not overall) length multiplied by 1.34. Generally speaking, the resulting number is the fastest in knots a displacement hull can go, regardless of the amount of horsepower applied to it.
In praxis, it depends on circumstances, a boat can go a knot faster, but it doesn't depend on horsepower. Horsepower is valuable, for instance, in bad weather or stream...
I have Beta Marine 43hp, practically 40hp.
In praxis, it depends on circumstances, a boat can go a knot faster, but it doesn't depend on horsepower. Horsepower is valuable, for instance, in bad weather or stream...
I have Beta Marine 43hp, practically 40hp.
Re: 28' Trawler (TW28) build boat speed
When you select a propellor for a displacement boat, you choose one for thrust, not one for speed like a planning boat. Boats like the TW28 are influenced by wind and current and must be able to have the HP to power a prop big enough control those two conditions, thus the designers indicated 45HP requirement.
Tom
Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978
Re: 28' Trawler (TW28) build boat speed
ok, so the 40/45 HP engine won't increase the speed of the boat, but it will make it easier to handle in bad conditions and weather as it will have more force to push against the wind and waves.
thanks.
thanks.
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Re: 28' Trawler (TW28) build boat speed
1.34• root(28) = 1.34•5.29=7.09ks or about 8mph is what to expect
Ought to deliver exceptional mpg as well.
Ought to deliver exceptional mpg as well.
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Re: 28' Trawler (TW28) build boat speed
You will find the boat to be the happiest at 1.0-1.2 of hull speed. So figure around 6 knots for real world speed. Displacement speed is a product of water line length. That is how an aircraft carrier can go so fast. 900 foot water line= 30x 1.34=darn fast
Re: 28' Trawler (TW28) build boat speed
I hadn't realized that the boat speed was dependent on the water line length. I'm curious about the physics of that. I always assumed it was mostly a matter of weight.
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Re: 28' Trawler (TW28) build boat speed
Wetted surface area functions differently for displacement vs planing boats. I’m not fully able to understand the why, but the simple rule is “long and thin is fast for displacement”.
For planing boats it has to do with the wetted surface being reduced at speed. Weight increases the wetted surface by sinking the boat further in the water.
Semi displacement hulls make a mess of everything in my head. Both rules apply at different times and at the same time.
Luckily for the universe I’m never designing a boat so I don’t need to know more than this.
For planing boats it has to do with the wetted surface being reduced at speed. Weight increases the wetted surface by sinking the boat further in the water.
Semi displacement hulls make a mess of everything in my head. Both rules apply at different times and at the same time.
Luckily for the universe I’m never designing a boat so I don’t need to know more than this.
Re: 28' Trawler (TW28) build boat speed
Jaysen fast and long is not always good as it reduces stability. It has to be a moderate amount for a displacement hull. My calculators show a best displacement speed, usually in the upper 7mph range, and then the top speed.Jaysen wrote: ↑Mon Jun 27, 2022 9:06 am Wetted surface area functions differently for displacement vs planing boats. I’m not fully able to understand the why, but the simple rule is “long and thin is fast for displacement”.
For planing boats it has to do with the wetted surface being reduced at speed. Weight increases the wetted surface by sinking the boat further in the water.
Semi displacement hulls make a mess of everything in my head. Both rules apply at different times and at the same time.
Luckily for the universe I’m never designing a boat so I don’t need to know more than this.
As far as planning boats it depends on hull shape and also varies on HP. You are also correct that weight is a function in the calculation. You can go from hulls like a Cigarette to a DE23. Jacques has designed both.
Semi-displacement hulls are designed for planning at lower speeds at efficient fuel economy. Jacques Nina is a good example.
Design is a complicated computation and why Jacques uses design software, and I use software for speed and prop calculations.
Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978
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Re: 28' Trawler (TW28) build boat speed
For what it is worth, my understanding of the limits of the hull speed is as follow.
The distance between two waves created by the movement of the hull at hull speed is equal to the length of the hull.
At higher speeds this distance becomes larger than the length of the hull. The first wave is at the bow, the second behind the boat. So at speeds higher than the hull speed you would be trying to run the boat uphill as the boat is not able to go faster than the bow wave.
The formula gives an approximation and is dependent on the exact conformation of the hull.
The comfortable cruising speed of the TW28 in my hands is about 10 km/h or about 6.25 mi/h. You can push her with a 40hp engine to 8 mi/h, but the noise is not comfortable then.
I forgot the name of the builder, but I remember a Turkish build TW28 with an 80 hp engine and trim plates. It achieved higher speeds.
In my opinion, although I am a big TW28 fan, if you need/want higher speeds, it is not the best choice.
Presently there are new efficient hulls that get around the hull speed limit without planing. It would be interesting to have a look at those, although I will not build another boat anymore.
Peter,
LUS
I remember his name: Ilker and his posts are here viewtopic.php?t=18076
The distance between two waves created by the movement of the hull at hull speed is equal to the length of the hull.
At higher speeds this distance becomes larger than the length of the hull. The first wave is at the bow, the second behind the boat. So at speeds higher than the hull speed you would be trying to run the boat uphill as the boat is not able to go faster than the bow wave.
The formula gives an approximation and is dependent on the exact conformation of the hull.
The comfortable cruising speed of the TW28 in my hands is about 10 km/h or about 6.25 mi/h. You can push her with a 40hp engine to 8 mi/h, but the noise is not comfortable then.
I forgot the name of the builder, but I remember a Turkish build TW28 with an 80 hp engine and trim plates. It achieved higher speeds.
In my opinion, although I am a big TW28 fan, if you need/want higher speeds, it is not the best choice.
Presently there are new efficient hulls that get around the hull speed limit without planing. It would be interesting to have a look at those, although I will not build another boat anymore.
Peter,
LUS
I remember his name: Ilker and his posts are here viewtopic.php?t=18076
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