7.5m powerboat design i have been working on

[Robert-Shetland]

Hello all,

Just a quick post with a few rendered images of a boat design i have been working on now for a good while, designed in Rhino with Orca 3d and to be built in Aluminium. I have started the MacNaugton yacht design school course and one of the first lessons asks you to design your "Ideal Boat" so this was mine,
The boats particulars are 7.5m long, 2.5m wide, 1.1t Aluminium weight, 1550 with fixed equipment, fuel & no people onboard, and max weight with 8 passengers and equipment, fuel etc of 3095kg.

The boat with the Savitsky analysis with twin 140's or a single 300 will acheive up to 40kts with 2 people, or 35 with 8, will be rated Category B for 6 and Cat C for 8, I have put this here basically to hear your thoughts on the design, any opinions welcome.

As you will see there are 2 seperate designs as far as the cabin roof goes, just two seperate ideas i had.

Thanks
Robert

 

[fallguy]

How do you know the boat is balanced?

in terms of buoyancy and centers of mass?

 

[Robert-Shetland]

How do you know the boat is balanced?

in terms of buoyancy and centers of mass?

Hi,
I have run it through all calculations using Orca 3d and done manual calculations also, i have also had others with much more experience than myself in these things take a look at it to confirm the results i am getting are correct.

 

[fallguy]

Hi,
I have run it through all calculations using Orca 3d and done manual calculations also, i have also had others with much more experience than myself in these things take a look at it to confirm the results i am getting are correct.

Does the boat enter negative trim with the wheelhouse full of passengers? just curious if you checked, I like the design, but wondering if you had to modify the hull for the house .

 

[Robert-Shetland]

Does the boat enter negative trim with the wheelhouse full of passengers? just curious if you checked, I like the design, but wondering if you had to modify the hull for the house .

I have only checked this with up to 5 people at 100kg in the wheelhouse which i thought would be a fair number in there, and the trim stays ok at that, also have put all the batteries aft, and the fuel tank is slightly aft of mid also. Not had to alter the design as such no, and kept the weight down by using 3mm rather than 4 for the cabin and everything above deck.

 

[Robert-Shetland]

Does the boat enter negative trim with the wheelhouse full of passengers? just curious if you checked, I like the design, but wondering if you had to modify the hull for the house .

Just tried it, to get the boat to go into negative trim, it would need 600kg's of weight in the wheelhouse, that would be the deck level, so actually over 600. quite pleased with that result actually.

 

[Robert-Shetland]

I am myself a beginner in motorboat design, I tinkered a formula to estimate the peak speed on calm water in function of the aft deadrise, the total weight and the power installed, calling it the "Crouch deadrise" formula, because based on Crouch one + simplified Savitsky analysis, and calibrated on few results collected here and there :
V = 1817 * ((0,087-0,0015 Beta))^0,5 * (P/m)^0,5
V = Speed in Knots , Beta = Deadrise angle (°), P = Power installed in Kw , m = weight in kg
In your case, assuming an aft deadrise of 10° (?) + a power installed of 280 to 300 hp and a weight of 3095 kg, that gives :
V = 35,0 to 36,3 Knots
With a weight of 2500 kg, that gives V = 39,0 to 40,3 Knots
>>> the orders of magnitude seem quite close to yours, but I also welcome comments and other in situ results.

Great formula, the deadrise at the aft end of this boat is 18 degrees, you are doing a lot better than me to work that out like that though, i'm just using the Savitsky analysis built into Orca 3d.

 

[fallguy]

Just tried it, to get the boat to go into negative trim, it would need 600kg's of weight in the wheelhouse, that would be the deck level, so actually over 600. quite pleased with that result actually.

I like the numbers, but I'm having a lot of trouble with the intuition. Probably because I have not seen the bottom.

 

[Robert-Shetland]

I like the numbers, but I'm having a lot of trouble with the intuition. Probably because I have not seen the bottom.

Here's a picture of the bottom from the bow, can get another from the stern if that helps.

 

[Dolfiman]

My post reworded :
I am myself a beginner in motorboat design, I tinkered a formula to estimate the peak speed on calm water in function of the aft deadrise, the total weight and the power installed, calling it the "Crouch deadrise" formula, because based on Crouch one + simplified Savitsky analysis to take into account the deadrise, and calibrated on few results collected here and there :
V = 502,56 * (0,087-0,0015 Beta)^0,5 * (P/m)^0,5
V = Speed in Knots , Beta = Deadrise angle (°), P = Power installed in Kw , m = weight in kg

In your case, with an aft deadrise of 18°+ a power installed of 280 to 300 hp (> 208,8 to 223,7 kW) :
** with a weight of 3095 kg >>> V = 32,0 to 33,1 Knots
** with a weight of 2500 kg >>> V = 35,6 to 36,8 Knots
>>> the orders of magnitude seem quite close to yours, but I also welcome comments and other in situ results.

 

[fallguy]

Here's a picture of the bottom from the bow, can get another from the stern if that helps.
View attachment 185558

Ah, yes, this helps, tbanks. There is a lot of hull forward underwater. I expect there is some speed tradeoffs, but I'm not a speed guy, so well worth it by me.

 

[Robert-Shetland]

Ah, yes, this helps, tbanks. There is a lot of hull forward underwater. I expect there is some speed tradeoffs, but I'm not a speed guy, so well worth it by me.

Yes the speed will be negatively affected, but not looking for speed, more interested in handling in poor conditions and stability, should be able to cruise at 20kts and that is more than enough unless you are in a loch