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Nauvoo
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| Joined: | Sun Oct 11th, 2009 |
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Posted: Sat Oct 24th, 2009 04:45 pm |
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I posted this at Boatdesign.net a few days ago, but nobody has replied (maybe for good reason?).
I have a 16' fiberglass skiff with a 40" flat bottom. It's a sharpie type planing hull much like a miniature Pacific City Dory, low freeboard, approximately 200lbs hull weight. The old (70's) 25 Chrysler it came with is in poor shape (needs at least a carb rebuild and a bearing in the lower unit), and makes the hull sit low in the water at the stern.
I happen to have a 6hp horizontal Briggs engine, which gave me the thought of mounting a motor amidships, building a shaft log, and running a 3/4" drive shaft to a prop. I'd also build a rudder to steer with, dory style with a yoke. This weight placement would get the stern up quite a bit, and probably help in other ways.
I'm aware that I'd have to deal with the exhaust in some way or suffer the fumes/noise consequences and I have a few ideas to develop. I'd use a pulleys/belt setup to get the power from the motor to the shaft and build a pivoting motor mount that would use the motor's weight to keep tension on the belt. This would also allow me to rig some kind of mechanism to tip the motor up enough to relieve the belt tension, effectively putting the boat in neutral. I realize there are no hard and fast answers, but my main questions are the following:
1) Might 6hp get the boat on plane with 2 people aboard? I don't have to fly; 12-15mph would do.
2) What kind/diameter/pitch prop would I use? I realize that it would involve some trial and error.
I also found a new Honda 13hp for a good price ($500+); the same questions apply.
I'd like to keep the whole installation under a thou, which seems doable with either motor.
All thoughts are welcome.  Last edited on Sat Oct 24th, 2009 04:45 pm by Nauvoo |
lastcreek
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Posted: Sat Oct 24th, 2009 05:37 pm |
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Back when, inboards were the way most hulls were powered, especially by the commercial fisherman. At the time outboards weren't reliable enough to be trusted. The setup is going to be noisy, and smelly. You'll need a transmission, which will add cost and weight, a packing gland, the rudder assembly, USCG approved flame arrestor on the carb, in hull exhaust pipe to get the fumes aft, or a stack. It'd be easier to hang a 6 hp outboard on the transom, and cheaper. As a project though it sounds like fun.
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Nauvoo
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Posted: Sat Oct 24th, 2009 05:51 pm |
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Money is the object here. 
But I do see your point.
It would be easier to fix the outboard and add flotation boxes (I've heard the referred to as "pods") to the transom on either side of the outboard.
The outboard, as mentioned, is a 70's unit. My concern is that it may eventually break something, probably when I'm several hundred yards offshore, and that the something will no longer be manufactured or kept in stock.
The smartest thing would be to add the "pods" to the hull and start over with a new 25, but I don't have the $3k cash. $1k I can swing, if I spend it as I build. Perhaps somebody has attractive financing?
I just want this boat to be working and dependable.
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lastcreek
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Posted: Sat Oct 24th, 2009 07:26 pm |
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Where I use my skiff, there is no need for speed.
I use a 5 hp nissan 4 stroke, engine weight 55 lbs,
pushes my skiff at 10 to 12 mph, burns a pint of fuel per trip.
If all you need is a kicker, and you're only going a few hundred yards,
sell the chrysler for parts, buy a late model used 6 hp and get on the water.
Safer, cheaper, more reliable than a 30 year old antique.
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Billsails
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Posted: Fri Oct 30th, 2009 03:56 am |
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| Hi : I use a Honda 2 hsp OB to push my 19' fiberglass Flying Scott sailboat . It weigh 850lbs + the weight of 2 crew & equipment . It gets me where I need to go against & with the ocean tides. Bill , Brooklyn NY
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Nauvoo
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Posted: Fri Oct 30th, 2009 12:38 pm |
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Bill,
I can see how your displacement hull would move nicely with minimal hp.
When it's not up on plane, a planing hull like mine will drag water behind it. Hp is the key to getting on plane, but the variables of weight, drag and wetted surface all play a part in how much hp a planing hull needs to plane.
I was kind of hoping to get some input on using an air-cooled engine as an inboard, as well as opinions on how much hp a boat my size would need to get on plane, even if just barely.
Lastcreek,
How big is your boat? What does it weigh?Last edited on Fri Oct 30th, 2009 12:38 pm by Nauvoo |
Nauvoo
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Posted: Fri Oct 30th, 2009 03:24 pm |
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Never mind, Lastcreek, I just found your boat.
I need more juice, as mine's longer and heavier. Probably at least 10hp.
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flyinwall
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Posted: Mon May 10th, 2010 03:10 am |
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just letting you know that there has been a thread on the wooden boat forum for a while about doing this very thing here is the link
http://forum.woodenboat.com/showthread.php?t=109868
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EIIShayneWard
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