Full Displacement Hull Question

[rocks1664]

Hi all

I'm new here and to the water so please excuse me.

I have a 24 ft Tamar 2000 fishing boat with a full displacement hull. I understand the speed of the boat is governed by length, width etc and only expect say 7-8 knots. my problem is when i give her full throttle the bow points at the sky, the stern squats down and she will go along like that if I'd let her or possibly start taking water at the rear.
At mid throttle she seems to run fine with a large smooth wake but I am not sure of exact speed
My question is can I fit something to the hull to utilise the power more efficiently
The engine is a Perkins 4108 with a Thamesway Marine 2-1 Gearbox.

I have work to do on the 4108 and on reading this site common starting issues etc but I am concerned with the hull squat.

Regards

[btrayfors]

Hull speed is about 6.4 knots if the waterline length is 24'. It takes about 1 SHAFT HP per ton of displacement to push her to hull speed in quiet water. The 4-108 should be plenty powerful for that task.

Not much you can do to improve speed. It would take a ton of HP to push her up on plane, even if you could.

Be happy with her displacement hull, and get used to the 6 knot speed :-))

Bill

[David M]

Lets assume your length at the waterline is 23 feet...which is close enough for government work.

The square root of the length times 1.4 is 6.7 knots. That's your hull speed. Trying to push a full displacement hull much beyond this speed just wastes fuel and makes King Neptune mad for trying to break his rules. That's what Neptune does as a punishment for not listening to him...points your bow high and squats your stern. No amount of fannageling with your engine or drive train or putting in trim tabs is going to change his mind. He's a stubborn old coot that King Neptune.

[rocks1664]

Hi btrayfors

Its not an improvement in speed I am happy with that, its just if I push her it lifts the bow up into the air, and I do mean up into the air, its this that I am concerned with. Or do I just not rev the engine that high.

Cheers

Ian

[rocks1664]

Cheers chaps

I wont push the lever so hard so it wont happen

Thanks again

[David M]

If you're engine is even capable of pushing the bow skywards, the engine is overpowered for the boat. If you could see the difference in your fuel consumption between running your boat at 6.7 knots versus 8 knots, you probably would not run the boat at 8 knots.

[rtbates]

Trim tabs is what you need. Basically flat plates off the stern parallel with the water. They move just like an airplanes elevator and sets the stern up or down and the bow opposite, ie stern up/ bow down and visa versa.

[svHyLyte]

Randy--

Trim tabs will be of little use. The yacht will attempt to ride up her bow-wave dropping her stern into the trough. The tabs will raise the stern modestly but the resulting drag will slow the yacht causing her to slid down the back slope of the bow wave. More fuel consumption with no greater speed. The dead-rise angle on the hull and resulting wetted surface area is too great to allow it to plane so why fight the physics?

[David M]

Exactly, a planing vessel has a minimal deadrise angle in the aft section. It also has a sharp turn of the bilge. (hard chine), especially at the base of the transom.

You may have to buy a Boston Whaler.

[Solitude]

Trim Tabs are for planing hulls. The best advice I see here is to throttle back. Let the boat tell you when its happy. Smoother ride, less fuel, and being able to sit at the helm without holding on for dear life.

Save the extra HP for when your battling wind and seas.

[Laidback]

Originally Posted by rocks1664

Hi all
I have a 24 ft Tamar 2000 fishing boat with a displacement hull.
The engine is a Perkins 4108 with a Thamesway Marine 2-1 Gearbox.
Regards

Hello,

As others have mentioned the engine may be over powered for the job of moving your boat at its hull speed.
Tamar 2000 LWL 22.5ft = 6.37kts.
The standard engine fitted is usually around 35Hp MAX at 3000 RPM.
Your Engine produces 51Hp at Max 4000 RPM.
So, depending on the propeller's diameter and its pitch, when you put your foot down , think of the analogy of doing a "wheely" on a powerful motor cycle - too much throttle = bike wants to fly.

With all that extra power available , you should be able to comfortably achieve your hull speed at around 2000 RPM. Possibly a prop of a different pitch should be looked into.
If your engine replaced a lighter engine at some stage, then this could also mean that the extra weight in the rear of the boat has altered the dynamics of the way she sits in the water under full power.