Yes I know, I have too much time on my hands.

So in my never ending quest to do the unusual or just plain silly here's the "idea". I have amongst my languishing herd of small diesels a Universal M25XP marine diesel rated at 23hp@3200rpm which is in fine shape. I have been offer a free (yes I know there's no such thing as a free boat, call it cheap) 1978 Chris Craft Catalina 281 which is in OK shape except for the inoperable V8 gas motor (you see where I'm going here already don't you?). I have a variety of transmissions for the Universal including a Technodrive TMC60A 2.5 to one 7 degree down angle transmission. The boat with the gas motor weighs about 5000 lbs. so the engine has plenty enough to push this thing to displacement speeds, which is all I care about, 5 or 6kts is fine. What I am looking for is a reasonably spacious and livable boat to do some long distance inland cruising in places like the Erie Canal and portions of the Great Loop. I measured the engine compartment and I can work on the engine from a lawn chair and there's room for at least 100 gallons of fuel. Think of the range? I'm not concerned about resale as there probably isn't any and as I currently own four boats obviously I don't sell them anyway, plus I need a low buck boat project to keep me sane. Anyone have any experience with something like this?

 

So what kind of drive is on it ? Sterndrive ?

 

It's a straight inboard.

 

So you intend to retain the shaft drive ? I suppose you could just use a low pitch propeller. Does/did the boat handle OK at slow speeds in its current configuration ? I mean, it sounds like a deep vee, and sometimes they wander about when off-plane.

 

I've never actually been on the water in this boat and it is a deep Vee, I'm aware that the smallish rudder makes handling a little odd but I don't know about tracking. I have a larger rudder that may fit it to aid handling. A 2.5 to 1 gear will use about a 15 or 16 inch diameter prop of which I have several of with a 12 to 15 inch pitch if they'll fit, I do have a couple of 14's also. A large part of the attraction is I have on hand much of what it may take to accomplish this. I've seen hundreds of cabin type cruisers on the Erie canal where the speed limit varies between 5 and 8kts for most sections so slow is all that's needed. What I'm looking for is the interior space with a decent head, sleeping accommodations and a workable galley for one, along with range, reliability and economy.

 

How many horsepower will it take to push your vessel to desired cruise speed?
You probably should have some ponies in reserve, and it might be a little nerve wracking to run the power plant at WOT to achieve cruise speed.

 

You've got all the stuff, just do it, I'm sure it'll cruise fast enough.

 

Classic hull speed for a full displacement hull typically takes less than 1 hp per 500 lbs., as the boat weighs about 5000 then 10hp should move it to hull speed, or close leaving 10 or 12hp in reserve.

 

It would work ok, you probably need more then 10hp to push her to hull speed, planing hulls are not really efficient at displacement speeds, but it's not a real problem on canals.

 

Guthunch says you'll have trouble in 10 knots of wind. I'd say you'll find it to be okay with the wind pushing, but every other way will be rough. Propping as big a prop as possible will help. Small props exacerbate the issue.

Planned use is important. The boat won't do well in big seas or much wind I'd say and won't have lotsa reserve.

I have a 2.5 Yamaha on my 16' skiff and it has this exact issue. The weight of the boat is probably 1800, so you gain some on the simple weight issue, but the cc281 is subject to a lot more windage.

So, I'd say, right prop, right conditions, it'd work. But one meter fast seas and wind is a harbor run.

 

A few knots of current and a stiff breeeze will restrict you to the dock.
Don’t forget drivetrain losses, lack of keel for directional stability, fixed prop range limitations, etc.
A big prop is called for, but prop walk will be a factor in dock handling, and crosswind situation.
Your speed/weight calculation is based on ideal conditions, not often found out and about in the real world.

 

Won't this boat have the same directional stability at 6kts regardless of horsepower? I like prop walk, it's very handy when handling a boat if you know how to use it, as are springlines. I'm aware I used ideal conditions for my calculations, what else could I have used as a starting point?

 

How about showing us the hull, especially the underwater portion?
Vague information produces vague results!
The hull will have the same characteristics it always did, but the bigger propeller will make a handling difference.
Remember that you’re always going slow, so prop walk may always be present.
Sure, you use ideal conditions for a starting point, that’s the data available.
I’m suggesting you factor in some leeway for less than ideal conditions.

 

Prop walk is like having a stern thruster, that works really well in only one direction.
Get used to it, and you'll look like a star!