Planing hulls and deadrise

[Mr Love]

I have a question as to how much influence deadrise has on fast planing hulls,specifically sailboards.
I am talking about very small deadrise angles.
eg 40cm wide ,flat rocker,planing flat length of 60cm.
Deadrise angle of 0.6 deg compared to deadrise angle of 1.2 deg.
Will there be any decernable difference in top speed on flattish water?

[gonzo]

The more angle you have the more power it will need to plane. Deadrise helps for handling and increased directional stability.

[lewisboats]

at those small angles...you would only be able to tell in a water tank if even then

[Mr Love]

Thanks for the input. This is a speed board designed to go as fast as possible. I included some V for control as the water is never glassy flat in the 25 knot plus windspeeds this board will be running in.
My concern was that 1.2 degree deadrise angle is more than most speed boards run, 0 degree to 0.6 degree being the norm. Is this going to be detrimental to top speed assuming that I am fully planing?

[lewisboats]

I doubt it...but I am no expert on speed boards...they may be really touchy about it. I hardly thing that 0.6 of a degree will make a huge difference though.

[Mr Love]

Thanks lewisboats, I have spoken to a few board design experts today and have drawn the same conclusion that it will make very little difference. If there is any loss of lift it can certainly be offset by fin lift ie slightly more upright or slightly softer fin.
Regards Martin

[Joakim]

This is a very interesting subject. If you were designing a small speed record racing boat for speeds about 50 kn, it would have a rather deep V. That is due to the fact that flat (and low V) bottoms are more prone to porpoising and thus you are not able to get the bow up, which really helps to reduce the wetted surface, which is the most important drag component at those speeds. Also deep V reduces nicely the width of the wetted hull.

Here is a boat going close to 50 kn with a 25 hp outboard: http://www.bergshamravarv.se/rekordf...0071006-45.jpg

I have done quite a lot of windsurfing, but only up to about 20 kn. Is porpoising ever a problem at higher speeds? What is a typical trim angle?

Joakim

[Mr Love]

Hi Joakim, When speed windsurfing you are running in high windspeeds,generally 25 knots minimum and ideally 30 knots plus. Therefore it is almost impossible to find dead flat water. Due to the chop the trim angle is constantly varying as you bounce over it.
Porpoising is a factor limiting your top speed as you are trying to unwet as much of the board as possible to reduce drag.However the opposite is generally the issue, getting airbourne due to too much nose lift. It,s a fine line.
I beleive some deadrise is critical for control ,but the question is how much? Typically most boards run very,very little maybe 0.5 deg max. Mine has 1 deg, thus the question regarding what negative concequences that may have.
After the feedback here and some other advice I have had, I am now convinced that 1 degree is fine and may even have some advantages, it will certainly handle chop better.
There is also another factor on a sailboard that powerboats don't have and that is the fin. This generates lift and has a large effect on trim angles. In fact the fin is probably the most important factor influencing the top speed of sailboards and the work that has been done on fins in recent years is why boards have now reached peak speeds of 50 knots, unheard of a few years ago.
Yes it,s an interesting subject and I welcome the input I have received.
Regards Martin

[Joakim]

Has anyone ever tried a deeper (at least 10 degrees) V? I think it might be worth a try. Flat bottom is faster at lower speeds, but not anymore at high speeds. With deeper V bottom you will need to have spray lists and "extra chines" to keep the wetted area at minimum and sprays of the hull.

Joakim

[Mr Love]

I really don't know and it,s an interesting thought. Sounds like I have another project

[Commuter Boats]

I'm not a board sailor but I would guess that while sailing the board is not level and any dead rise will present a flatter section to the ocean.
Bolger is an advocate of flat bottom, plum sided sailing boats, claiming that they provide a soft ride once healed.