difference 15/20 degree V

[Jasper_ghost]

I consider a 26 feet cabin cruiser aluboat (precut) trailerable mainly to be used inshore and in calm to medium rough sea and powered by a 300hp outboard. Purpose is diving and fishing so also some anchering and trolling. Please comment or correct me if im wrong

So far i think a 20 degree hull is a good choice to give a smooth ride in medium rough sea. But Im looking for experience/good advice with 15 degree contra 20 degree design. What is the pros/cons with regard to:
stability
planning/cruice/max speed
mpg.
other

Will 20 degree be unstable when anchoring, and how important is it to keep COG low.

Thanks,
Jasper

[Gilbert]

Sorry to say I do not have the exact data that you are looking for but I can give you my recommendation.
I would say you have chosen a very nice size boat for the purposes you mention. A 26' boat will make a reasonable speed while not planing in very bad conditions. A 15 degree deadrise bottom will give you a lot higher feul efficiancy than a 20 degree deadrise. The top speed you can achieve with 300 hp will be much higher also. Personally, I would prefer even less than 15 degree deadrise to gain even more efficiency and the potential for more top speed, but you did not ask about that option. And with all this I must say that the deeper veed hulls can provide a quite smooth ride in some pretty nasty chop, but they too will have to slow down at some point as the conditions worsen.

[Jasper_ghost]

good point, tks. Does any have a rough idea of the % increase in mpg or top speed for the 15/20 degree?

[fastwave]

If you consider the change in V changes the vertical component in hydrodynamic lift then the difference is only 3% in lift from 15 to 20 deg.

[Gilbert]

If you are suggesting that a boat with 20 degree deadrise would only use 3% more fuel than an otherwise similar boat with 15 degree deadrise, well, let's just say I am not believing it. I have been expecting someone to step up to the plate with some actual figures here, but so far no one has.

[fastwave]

I am not suggesting that it is 3% more speed. I am saying that in the most simplistic case there is 3% less lift. That might translate in 5 or 10% more drag or whatever. Which in turn might translate into more speed difference.

[Guest625101138]

All other things being equal (unlikely) the deeper "V" will use around 7% more fuel. It will probably use more than this because you will be able to push it harder in rough conditions. Other factors that could outweigh the deeper "V"t o consider are overall weight and cabin windage.

The "V" is not the sole determinant of ride comfort and control but it certainly reduces slamming.

Normally a 26ft boat with a large outboard and fuel plus gear will sit near or below the chine at the stern so the deeper "V" does not alter at-rest stability too much. I remember the early Bertram hulls had a tunnel in the "V" that filled with water at rest. This was only in the 15ft hull to my knowledge and it probably floated higher than the chine if the chamber was not flooded.

Deep "V" hulls have a tendency to lean into cross-wind so it might need trim tabs to correct roll.

You should get some experience with you preferred hull in the intended conditions before committing. My information is a bit dated as I refuse to provide further support for fuel rich countries so use other forms of propulsion these days.

Rick W.

[RANCHI OTTO]

Calculate using Savitsky the bare hull resistance for a boat like this..

Lwl = 12 m
Bwl = 3.5 m
Lcg = 4.5 m
Displ = 8000 kg

Deardire = 15 deg or 20 deg.

speed range btw. 30 and 40 knota

SURPRISE...SURPRISE...:p

at 30 knots speed the deadrise 15 deg. has 6.6% less resistance but at 40 knots 6% more....
even if you modify the L.c.g. the differences are low...

[Jasper_ghost]

Wow, thanks
I belive most models is calculated in calm water, but most of the time I will be in some waves riding with 25kn. I belive each wave for a 15 degree hull will absorb some energy where the 20 degree hull will make a "cut" as well as being more comfortable.
My hope is to have a mpg in the range 1,5-2 but is this realistic for a 20 degree hull, 26feet, beam 8 and and total load 2500kg and a suzuki df300.

The hull will have delta pad. I intend TIG welding below waterline and in level with hull and also be polished for best mpg. Any other good advise to consider for such a project.

[Gilbert]

Surprise indeed that a shape that generates less lift than another would achieve less resistance as the speed increases. Forgive my insubordination, but this sounds like a mathematical error to me. Please explain how this magic is made to happen.
If this is true, I believe that all the unlimited hydros would have deep V hulls. And the deeper the better, I'm guessing. And obviously they have nothing of the kind.

[RANCHI OTTO]

These are results according to D.Savitsky..
The dynamic trim angle makes the differences...

[rwatson]

What is dyanmic trim ?

[Nojjan]

Ranchi Otto - Would you mind declaring your calculation?. Apart from being rather odd values (LCG too far forward and the hull is too wide for it's weight, at least for a >35 knot boat) I can't duplicate your calculation. I do not get an intersection on bare hull resistance from Savitsky only based on the two deadrise angles.

Rwatson - Dynamic trim is the angle of attack for the theoretical prismatic hull planing on the water (normally taken as water surface to 1/4 buttock angle).

[RANCHI OTTO]

Nojjan,

here they are the results of the calculation:

Deadrise 15 (20) deg.
--------------------
vk 30 > EHPbh = 217 (231)
vk 40 > EHPbh = 367 (357)

concerning the dimensions of the "virtual boat" there are not far from an existing one.

[RANCHI OTTO]

Nojjan,

here they are the results of the calculation:

Deadrise 15 (20) deg.
--------------------
vk 30 > EHPbh = 217 (231)
vk 40 > EHPbh = 367 (357)

concerning the dimensions of the "virtual boat" there are not far from an existing one.

[Guillermo]

Surprise indeed that a shape that generates less lift than another would achieve less resistance as the speed increases. Forgive my insubordination, but this sounds like a mathematical error to me. Please explain how this magic is made to happen.
Gilbert,
For a given deadrise angle there is an specific relationship between trim angle and lift coefficient which defines the inception of porpoising. Because of such porpoise considerations it is necessary to design the boat to operate at an unfavourable trim angle from the point of view of resistance, as the minimum drag-lift ratio occurs at an angle greater than the angle needed to avoid porpoising.

The trim angle required to avoid porpoising increases with increasing deadrise angle. Increasing the deadrise enables a planning surface to operate at trim angles more closely approaching those required for minimum drag.

So, it may happen a 20º deadrise brings better drag results than a 15º one at high speeds, if it risens the trim angle to a more efficient one from the point of view of the drag-lift ratio.

I attach the well known 'Savitsky 64' paper.

Cheers.

[Guest625101138]

.........My hope is to have a mpg in the range 1,5-2 but is this realistic for a 20 degree hull, 26feet, beam 8 and and total load 2500kg and a suzuki df300..........

Savitsky calculation indicates you need around 120HP for a boat with total weight of 2500kg doing 26kts and 60% overall propulsion efficiency. You could expect that performance will be similar to the boat shown here with your selected outboard:
http://www.suzukimarine.com/boat_builders/boat_tests/key_west_boats_inc/key_west_268_cc/df300/

I estimate your top speed will be around 37kts so a bit slower than the hull tested. Likewise a little more fuel at the various speeds. The tested hull is wider so should be a little more efficient. It has 21 degree deadrise so similar in this respect.

Rick W.

[Gilbert]

Very good!
This is half the information I was hoping to see.
Now do we have similar data for a boat that has about 15 degree deadrise and similar length, beam, weight and the same type of motor?