Displacement boat - speed

If he'd done it by changing weight distribution I'd agree, but I wouldn't make that assumption when adding a sponson/extension like this one.

 

The decrease in the trim, by itself, should improve the speed (we must assume that this type of ship has been designed so that its maximum speed is with zero trim) but we agree, the sponsons, but improve trim, increase resistance by friction.

 

Why must we assume that? The boat appears to have a traditional hull shape. It is not obvious to me that anyone consciously thought about the relationship between trim and speed when the boat was designed.

The sponsons/wash boards/giant trim tabs may also create resistance by wave generation.

 

But he is getting the same WOT rpm on idle and loaded, thus hitting the rpm limit may be the cause for no change in speed.

 

Yas, I think so, and that is the reason why I plan to increase pitch.

 

You're always so insightful. Thanks for your comments, always, help clarify the issues under discussion.

Yes, you are right, we don't must, we can, because it is normal to assume that.

Sorry as I totally disagree. All designs, with ships of this type are made with zero trim. This is really a must.
It is true that, when working with load conditions, the trim is usually non zero, but nothing more.

That is entirely debatable. Turbulence, strong flow disturbances, but no waves are produced.


Thanks again for your kind comments, which allow us to carry out this friendly exchange of views.

 

I agree, this is a perfectly reasonable assumption.

So you could have avoided the sponsons altogether, by trimming with weight distribution, and changing speed by playing with your propeller.

 

I tried to solve problem by trimming with weight distribution, but it was impossible. Now I solved this problem, but I need to calculate max pitch for this engine and this diameter of propeller.

 

There's another thread for that: http://www.boatdesign.net/forums/props/propeller-size-53373.html
It is a good practice not to discuss the same issue in two different threads, in order to avoid scattered and confusing replies.
Cheers

 

Duco84 not only added lift but extended the waterline lenght without a speed increase, I didn't hear about someone who could gain more speed with similar devices from a displacement hull, at least you have a swimming platform, did you consider a pair of trim tabs as they are adjustable and with less drag for dinamically trim changes? Hydrofoils that i have seen has dinamic lift also at the bow, it is better to increase trim than the opposite, especially at higher speed

 

Whit this engine I didn't except big increase of speed, but I excepted 1kn. Now trim is zero, but probable drag is to high, also I have big wave on bow.
What is Your suggestion, to change spoiler with trim tabs?

 

Trim tabs are for faster boats because they are fixed on the transom, you could take outboard unnecessary weights, change the glasses with policarbonate, add two spray rails about 10 cm wide starting upon the bow wave end extending aft around the stern, change the propeller and upgrade the engine but don't expect too much knots for the power that you install, because you boat has a displacement hull, safe and economical but slow.
Maybe you can find a semi-planning boat with faster specifications

 

Yes, displacement hull, but what is with menorquin boats?

 

A mighty Menorquin C120 does 14 knots with 2x285hp, their beam is 3,90 mt, fuel consumption is not mentioned on their website but I don't think that it would be very efficient, the two boats are not so similar!

 

I compared these two boats and I think they they are very similar. Boa=3.90m, same as on my boat. Also 14 kn is service speed, max is abt 18. Of course I'm sure that consumption is to high.