Novel boat drive - engine in the front

[amorrison]

Generally:
1/ Aircraft have center of mass in front of center of drag for stability . . .
2/ Boats have engines in rear thus center of mass is behind center of drag

but not in this boat ? ?

http://www.jdfab.com/jbarritt/hotboat.htm

Comments please.

 

[MikeHalloran]

That's how they made speedboats when a Liberty engine was the hot setup.

Shallow lake? I.e, no flotation required?

Looks like great fun.



Mike Halloran
Pembroke Pines, FL, USA

 

[HRHtex]

When you wrote "novel" and "front engine" I thought you were going to describe a boat with its propeller on the bow. These were tried in some speedboats in the 1930s - presumably they were non-planing hulls.
With your boat I don't think you can have the engine weight so far forward and still have a conventional planing hull.

 

[patprimmer]

It will get very interesting when you suddenly lift the throttle.

Aircraft travel through one medium only. Boats travel on the air water interface so the lift drag characteristics are very different on the top and bottom surfaces.

Regards
Pat
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[MikeHalloran]

It's a motorcycle engine, right?
Weighs next to nothing, as engines go.



Mike Halloran
Pembroke Pines, FL, USA

 

[Gearloose1]

Looks to me like matter of convenience, so they do not have to fit a V shaft to keep engine in rear / toward center.

 

[someguy79]

Depending on the engine used (motorcycle type) it might only wiegh 100 lb or so. That's not really a problem if you have someone driving from the rear.

I'm mostly concerned with the exposed driveshaft to catch your feet on.

 

[patprimmer]

Floorboards or a simple cover like they use in V dive boats solves that quite easily.

Weight distribution and shaft angles and line of thrust vs centre of drag and centre of lift at various speeds are much more of a concern.

Regards
Pat
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[HRHtex]

If you are using a motorcycle engine in a boat you can have a chain drive from the engine down to the propellor shaft running under the engine - this also allows a shallower angle on the shaft. I have seen this arrangement a few times on small hydroplanes.
Which reminds me - three-point hydros often have the engine well forward above the sponsons -although rear-engine hydros seem to be increasingly popular.

 

[patprimmer]

A three pointer is delicately balanced with most of the buoyancy in the sponsons to support the engine weight when stationary and the larger planing surfaces running from just behind the centre of gravity to well in front of it so the front lifts first to keep the full propeller buried until some speed builds then as the speed increases the propeller shaft lifts the rear until tit is planing on the third point being a small flat on the bottom of the skeg. This third point keeps the propeller at just the right height so it is fully surfaced with only the bottom half in the water and the propeller shaft and all but the planing surface of the skeg out of the water.

If the shaft angle is to steep it drives the nose down, if it is to shallow, it drives the nose up.

If the shaft axis passes through the hull to far forward, it lifts the nose, if to far back it lifts the tail.

The gap between the floor, the inside edges of the sponsons traps air and creates a ground effect. Trim angle of the floor determines mount of aero lift and longitudinal position of the sponson in contact or very close to the water surface controls where the lift applies.

If hydrodynamic lift is to great at the front the front lifts rapidly then stalls and falls, creating a porpoise like motion. Same if there is unstable suction at the back.

If aero lift is to high at the front the front climbs then as the angle of attack increases the lift, especially at front increases until the boat kites and becomes airborne and comes down upside down from a back flip or barrel rolls. These are the shots that make the news.

On a race boat you want the maximum aero lift you can get without exceeding the boat weight, so you maintain a minimum safe weight on the water surface. Waves and head winds both work against maintaining that balance and that is the razors edge of flying these machines.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[MikeHalloran]

All good stuff to know, however the extant boat appears to comprise a tube frame around the driveline, separated from the water by what is basically just the skin of a small monohull runabout, with one seat near the transom and no discernible flotation. It has what appears to be a rudimentary dry muffler, and padding on the seat, its only concessions to civility.





Mike Halloran
Pembroke Pines, FL, USA