outboard vs inboard

[duluthboats]

Will,
Thanks for putting the numbers down so we can understand them. It's a good illustration of how difficult it is to do comparisons.

Also how easy it is for me to get side tracked. The boat I started this thread with, would be overpowered with this rotary engine.

Are there any other thoughts on O/B vs I/B?
Gary

[tom28571]

Will, Gary & all,

Your posts bring up another important point. It is not just the top speed that we need to be choosing. The performance over the entire range may be even more important to most people. This point is well illustrated by Gary's boat. It is very fast, probably much faster than is needed for a boat intended for cruising and the fuel use figures look very good to me for such a heavy boat. My question is, how does it perform in the transition speed range between displacement and full planing and what is the extent of that range? Gary can answer that question easily.

Although "Liz" only needs 50hp to go 22mph and runs and rides easily at ANY speed in between, some compromise was made to do this. I do not know how much power she would accept or how fast she would go with more power. The compromise in design that was made in the design of the hull bottom makes it possible to run very level with no stern squating as planing is achieved. Unlike all the books predict, the trim angle increases throughout the speed range and is greatest at top speed. I know that top speed would be increased if I could force the trim a bit higher to decrease friction drag, but the longitudinal stability of the hull is too high to allow that with the current setup.

What I have done is to spread the lift further longitudinally than is normal by the chine flats which are at a positive angle of attack relative to the rest of the monohedron planing surface. This positive angle also caused the flats to get progressively wider toward the stern so that their maximum lift is at the the stern. This is my one original fundamental contribution to the design. Actually, I got the idea from thoughts of Weston Farmer from over 50 years ago. Anyway, I compromised some top speed in favor of a better cruising speed range. These chine flats are intended to work like trim tabs to help the boat get onto plane early. The difference is that they don't have the high drag of trim tabs and are, of course, fixed so they can not be lifted as speed is increased.

Apparently, the hydrodynamic lift of the hull does not overcome the lift of the chine flats until a much higher than normal speed is reached, thus the trim angle increase comes much later in the speed range than for "normal" boats.

I don't know how fast "Liz" would safely run with more power but I expect that at some point the tendency for the chine flats to depress the bow would cause handling problems. By using an overpitched prop, I did get up to 25mph and had no problems in smooth water. Probably 30mph or so would not be a problem in relatively smooth water. The chine flats also cause the boat to turn flatter than a normal V hull. Higher power might also cause problems with quick, high-speed turning. To test the safety, I've run through tight high-speed turns but as a caution would not try to treat her like a sport boat.

Designing planing boats is much more demanding than displacement or semi-displacement types and I have much to learn.

[duluthboats]

Ok, to continue this, I’ll be more specific.
LOA 9m
Beam 2.5m
Displacement (using Tom’s definition) 2000 kg
(Displacement = full cruising weight including all cruising stores, gear, fuel, water and crew.)
Cruising speed 16 kts
Top speed 20 kts
These are targets for my boat. Not to be confused with Option 1, which is still up for debate.

Gary

[Willallison]

Tom,
I assume you were actually referring to my boat, not Gary's, but the answer to your question - as I'm sure you already know - is it doesn't perform at all well in the lower speed range. Below about 22 knots the boat wants to fall off plane. Even with 400hp on the back, it struggles even to semi-plane. That is as a result of the way the the whole rig is set up.
But then, that isn't what the boat was designed for - the hull is an adaptation of an early Formula deep-vee, it was built by a somewhat infamous abalone poacher (not me ) so was built in order to carry a heavy load of illegally caught fish as fast as was possible. When we bought the boat (with two older carburetted o/b's which incidentally used almost twice the fuel..) it would do 60mph but would struggle to remain on the plane below about 30. We've altered the engine set up somewhat, so in spite of having much bigger engines (3Litre as opposed to 2.5) we still run a top of around 60mph, but can maintain slightly lower speeds as well. The boat is now under propped in order to achieve this - we estimate that with a light fuel load (it can carry 800Litres), the motors lifted up a bit and higher pitch props we could pull closer to 75mph.

As I said before, I think we need to consider some more basic design objectives before we can decide on what form of propulsion we use. I'll start a new thread on this

[tom28571]

Thanks Will,

Yes I made a mistake about who's boat I was talking about. It's not a knock on your boat but it does illustrate really well the other end of the performance envelope from where we are interested for a cruising boat. A ride in your boat would be exciting, especially for one just recently lured away (at least partly) from sailboats. I wish we had more input from the other guys on just what speed range we should be interested in. No serious discussion of hull design can be started without that nailed down, or, at least, a desired range which might have to be modified by reality. In many areas where we would like to cruise, your boat would be restricted to low displacement speed by no-wake rules.

While the beam limit of 2.5m has been established, the most important hull dimension for a planing boat is the waterline beam at the hull CG. For one thing it's about the only waterline dimension that does not change at different speeds.

[Willallison]

Couldn't agree with you more Tom.
I'm not suggesting we should adopt anything like "Force Eleven" for our coastal cruiser - it makes an excellent coastal cruiser so long as your happy zip from one spot to the next when it's relatively calm. The boat's capable of belting along at 40 knots in just about any weather - its occupants are not!

[tom4660]

I BUILT A 29 FOOT SEMI DISPLACEMENT HULL WITH TWIN OMC v6 CROSSFLOWS THE TOTAL FUEL CONSUMPTION AT 3000 RPM IS 11GPH @ 23KTS .TO USE OUTBOARDS EFFECTIVELY YOU MUST MODIFY THE HULL BY ADDING A HOOK OR FIXED TRIM TABS.THIS ALLOWS HIGHER SPEEDS WITHOUT OVER POWERING THE HULL AND TRYING TO Plane. ALSO THE ENGINES TIMING AND FUEL CURVES MUST BE MODIFIED. WITH OUTBOARDS PROPS,AND WIEGHT ARE CRITICAL.DONT TRY TO LUG THE ENGINES THEY WONT LIVE LONG LIKE THAT.

[Willallison]

Tom,
Is the 11gph you quoted per motor? If not then you are getting good economy given the technology of your engines. I assume that the boat is fairly light......
I'm not sure that I agree with about the hook in the hull, or fixed trim tabs. There aren't too many outboard powered raceboats running around with this configuration. I have a 19 foot tournament ski boat which has a slight hook. It helps keep the boat on plane to quite low speed and has a very low angle of attack - both good for water skiing, but its top end is limited as a result.
Trim tabs are designed to be adjustable in order to accomodate differing conditions - whether they be alterations in onboard weights or changes is sea state. For instance, when running with a sea, it is desirable to have more bow up trim. A hook in the running surface - whether it be as a result of hull shape or fixed tabs - would prevent this.
I'm a little confused with your remarks about preventing the boat from being overpowered and planing, yet at anything over about 7knots, your 29 footer is on the plane (1.34 x sq. root WL length).
One thing I definitely agree with you about is overloading. This applies to engines of all variety, but outboards do seem to like it the least. It is not so much a problem of over weight as it is about over propping.

[tom4660]

will you are right the boat is on plane ,what i meant was that the hull needs a bow down approach . without the tab or hook the boat ran bow up even with the outboards trimmed down.This was more evident at high speeds.The 11 gph is at cruse (total between engines) this goes up dramaticaly with rpm. At 5500 the motors burn 24gph according to flowscans.I dont run the motors above 3400 anyway, the hull doesnt like the speed and i dont like buying the gas.

[mitch]

Hi Guys! I do like the disscusion. Have you thought about small diesel i/o s. Check out the new issue of Boating. They have an artical about Reggie Fountain s new boat. With tripple Merc 500HP I/O S verses Tripple Yanmar Diesels running suface drives. It is surprising as for top speed and for higher planing speed as well as for fuel economy Neal