Scale power

I have completed a 1/6 scale model of an 18 foot inshore fast fisher cuddy, semi deep-v (18°) and it is powered by a bauer-modelle 750 outdrive with a 1454kv electric motor (as recommended and supplied by B-M). With 3 cell, it achieved 10.1 mph and with 4 cell 15.0 mph. These were the maximums after much weight distribution changes, raising and lowering of outdrive, and outdrive trim. The model weighs anywhere between 3.5 and 4.5 kg depending on ballast. I would want the full size boat to do about 30 mph with 90-100 hp and I estimate it to weigh all up with fuel and people about 1000kg. Is this model power representative do you think? I am also building a "streched" 22 foot version, again 1/6 scale, but with twin outdrive installations.

 

When you say "do" 30mph, do you mean top speed, or cruise speed ? It sounds realistic as a top speed, you should get there, but as a cruise speed might be a touch high with that power. What is the chine beam, aft ?

 

Welcome to the forum Bob c

Well, there is a lot in that simple Q.

And there in is the issue.
As a pure 'model' to ship, that is easy enough to state yes or no.

For example, a 18ft = 5.486m by 1/6th = 0.914m model

So at a displacement of 3.5kg scaled up = 756kg, or 4.5kg scaled up = 972kg.

So, on this 'simple' numbers, then yes, it is representative, as a model, like for like.

In terms of power not so easy at you have stated 1454kv, this is not units of power. So, perhaps if you can clear that one up.
But for the sale of assumption if you meant 1454W or 1.45kW it would be 769kW for the 22 footer.

Then we come to, the actual representative side.

Unless you have drawn up a GA, allocated the weights for the boat, their locations (for lcg/trim with hydrostatics) and arrived at a full load weight that is consistent with the 972kg of said hydrostatics.... not possible to say. As that is just a model that is scaled up.

It would bear no resemblance to an actual 'design' of that size. Since there is no data, other than a model, displacement and power.

 

Thanks for your reply, 30 mph would be top speed. Chine beam aft is 6'9"

Thank you Ad Hoc, i have tried to match weight distribution (GA?), 185kg for 100hp outboard (minus weight of model motor/outdrive) 200kg for fuel, 180kg for two people etc etc and put the lead scale weights in the correct places. The motor produces up to 700w of power. Hope this helps you to help me. Thanks again.

 

Hi, thanks for your help Mr E. Chine beam is 6'9" (which is the same on the 22footer). This may seem wide but i want stability for fishing. 30mph would be the maximum.

 

Sorry i hope you both got my replies

 

That is quite beamy for an 18 footer, with the sort of weight you mention, 90hp would probably give you 25 mph cruise, but I think a slightly bigger engine around 120hp would give you 30 mph cruise speed, which is nice to have when conditions allow. That chine width will cater to a fairly heavy motor.

 

yes got your reply.

Do the comments above - assist you?

By GA.. I mean a drawing showing the plan and profile view of the whole layout/design.
From that you can assign LCGs and weights, or each item that is onboard the final design to see if it matches your hydrostatics, for draft and displacement, and model test data.

 

I assume that is a specification/rating. The power actually produced as installed and used in the boat. is undoubtedly different and may be considerably different. A major factor in determining the power actually produced by the motor are the propeller characteristics.

A Savitsky analysis should provide a usable estimate of speed vs power for this boat at planing speeds. It would be at least as accurate as trying to extrapolate from tests of model with a motor producing an unknown amount of power. Another way to estimate power required is to look at data from similar boats.

 

Thank you for your help