Repowering IO with small outboard

I am looking at buying a 21-24' hull with large cockpit and cuddy, removing IO and patching transom and repowering with a 4-stroke 9.9HP.

No, I'm not completely crazy, but am looking at a boat that will go 6-8 mph and only use a maximum of 1 gallon per hour at cruise rpm.

If any of you have kickers that you use for main propulsion, I'd love your inputs and opinions on these areas.

1. Is 800-1,000 lbs. a good estimate for the weight removed when taking out a Mercruiser or Cobra engine and outdrive?
2. If I remove the engine in a boat like a 4 Winns 23.5 Sundowner (which shows boat weight of 3,100 lbs (with IO)), is 6-8 mph a reasonable estimate of a crruising speed?
3. Any particular snags in rigging steering or controls or using existing fuel tank?

I'm a sailor and considering adding a low-powered powerboat just for making trips from Lake Michigan down the Illinois, Mississippi, Ohio and Tennessee Rivers.

I appreciate your responses.

Murph'

 

You'll get the speed you want, but if it's a typical powerboat shape, it'll handle very poorly at these speeds and a high thrust model of 9.9 would be prefered compaired to a regular one.

 

Not a suitable thing for travelling any distance, as mentioned poor steering an issue, and the boat will become a little tippy with that weight removed, too. You are dragging a submerged transom around which increases resistance considerably.

 

8 MPH might be a bit high for that hull.

 

The fuel tank supplied with the little engine is too small and you will have to buy a bigger tank.

Consider a more powerfull outboard because with 10hp you might not be able to go upriver. Fuel economy will be almost the same if you keep the same speed.

 

Intersting thoughts, but I don't quite understand.

If I remove 1,000 lbs within 3 feet forward of the stern and go back with 100 lbs at the stern, why wouldn't the stern displace less and the boat float higher at the stern?

Assuming that the steering is mostly a function of the thrust at the propeller, why wouldn't it be comparable to the IO by simply carrying a bit more rpm than the normal idle speed from the IO during docking manuevers.

Since the removed weight is from about 18" on either side of the centerline, why would "tippyness" be an issue?

You guys may be right, but I don't see the logic just yet.

Thanks.

 

Planing hulls with a pronounced vee-bottom are inclined to wander around off-plane, there being no effective vertical "fence" to assist tracking, a planing hull with little vee, will have the near vertical sides well immersed, to assist with that. You could add skegs to help alleviate that. Remove all that engine weight from your vee-hull, and there will be a decline in initial stability, as the bouyancy will be more concentrated toward the centreline than before. Any shifting of passengers will be inclined to tip the boat more than previously.

 

Removing 1000lb from the bottom is removing ballast, which helps with stability. The boat will also float down by the bow, where there is less beam. The average beam will be less with the accompanying loss of initial stability.

 

You will end up with a boat that has poor directional stability, strongly reacts both longitudinal and transversely to people moving around in it and looks a bit funny with the bow down.
And with the little outboard having a hard time to deliver the speed you want, don't underestimate the noise level.

 

Gentlemen:

I think you have me convinced that this is not a good idea.

The amount of expertise and cooperation on this group is awesome and appreciated.

Greg Murphy