Outboard for a displacement hull

I have an old 18' diesel launch that had a Volvo Penta MD1b diesel inboard (10hp) in it. The diesel is beyond repair, prop is badly bent and shaft is in poor shape as well. I'm thinking of converting over to a 4 stroke outboard as the transom could be easliy modified to have a outboard well, plus it will free up allot of space in the boat not having the diesel sitting there anymore.
My question is, I only really need 10hp to move at hull speed, but I was thinking of putting a 20hp Honda in it to have some extra power if I ever needed it. Is this reasonable or am I wasting money on a larger engine or should I actually be considering possibly even a slightly larger engine. The hull is a round bilge and probably has a displacement of 2600 lbs when fully loaded.
Thanks in advance.

 

10hp is all you need. The real consern is do you find outboard engine with propeller and rpm suitable for a slow boat like yours. Are you using the boat on the river?
Teddy

 

I plan on using it on our lakes here, but I had thought at one time of maybe using it on the larger rivers. But the current on most of our rivers here runs at 4 to 10 mph if not greater so I thought it wouldn't be practical if I had to attempt to go upstream.
By the way here is a link some pictures of the boat i want to but the outboard on.
http://www.watsonriverdesign.com/Boat04.htm

 

Allan,

Ted's right on both points. A formula often used for power required for hull speed is ( HP = Displ/ (22500/hull speed squared). Assuming 3000 lbs displ, to be conservative, and LWL of 16', you'll need only 4 HP at the prop to reach hull speed.

This spreadsheet devised by the Alberg 30 Organization, an owners club for a displacement sailboat, should be useful. Input the data for your boat, and it will give you power and prop recommendations. In your case it shows 6 HP, which is conservative and probably allows for inaccurate weight estimates and some bottom fouling.

http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/propcalc.xls

As Ted said, 10 HP should be plenty, and you won't get much benefit from more power unless you plan to operate with a very heavy load. His point about the prop is right, though. You'll want a motor designed for displacement hull use with the max possible gear reduction. A larger, slower, prop will give more torque and be more efficient. The spreadsheet is designed for a displacement hull sailboat, so the numbers should work for your boat.

Nice boat. Good luck!

 

I'm currently using a Yamaha 8hp 4 stroke hi thrust, elec start and power tilt . Big prop for an 8hp.

These are normally used as a trolling motor on large boats. You should investigate it, might work for you too.

 

Maybe 2x5HP ??

 

The 8HP high thrust should be a good choice. The electric start, remote tilt and small alternator will make it a lot more user friendly and relaxing - a gentleman's river boat.

The 8HP should give you reserve to push that big cabin into a decent head wind. You should end up with a lighter boat as well that is easier to launch. It will use a bit more fuel than the diesel though.

Also don't underestimate the convenience of electric start and battery for lighting and maybe bilge pump. It can give it a big boat feel without the high fuel cost of a big boat.

Rick W.

 

Nice boat! If I were you, I would place a couple of electric outboards, batteries, a solarpanel on the roof and drive silently around the lake. If needing occasionally more power for a longer tour, a lightweight genset gives the extra hours.. Anyway there's lot of light during the summer in Yukon, allthough not as much as here (2 months on a roll, 24/7) .

 

I had actually thought of electric but after looking at what was available, the cost as well as the weight that batteries would add to give me the range I wanted I decided not to go that route. It seems everyone is thinking that 10hp is more than enough, so does anyone have any thoughts on the Honda BF9.9 , long shaft, electric start with remote control as far as being a good choice for this boat. There's a dealer here and I've seen a few of these engines perform but none of the other makes in this hp range.
This is a question for Ted, I noticed you're located in Finland, I was wondering if you have seen many of these boats (Marina 75) around http://www.watsonriverdesign.com/Boat01.htm and if you have any local knowledge about them.

 

Here's one for sale http://www.nettivene.com/viewBoat.php?id_boat=262930
Actually I had take a search to find out, but finally recalled that it was originally Finnark 75 which was quite popular round 70-80's. Some of them, however not many, have had problems with faults in the laminate. Maybe caused by improper (=not at all) maintenance I think.
The Marina 75 is modernized version by new manufacturer of the boat. Nice, traditional Finnish design. Never been aboard one.

 

For the outboard you got get a propeller that's suitable for the slow speeds. For the 9.9:
http://www.piranha.com/enginelist.php?id=7843 Here the "slowest" propeller for the engine for speeds around 10kn. http://www.piranha.com/enginelist.php?id=7842 shows that for the 8hp Honda there's one rated for 2kn less speed so it's actually better choice methink...

Best to contact Distributor/Importer/Manufacturer for accurate recommendations. Sometimes it's worth it. And if you do, put their response here so we can "judge" it

 

Suzuki re-introduced a high thrust version of their 9.9 this year. They had it years ago with 2-strokes. It's all you really need and comes in 20 and 25in versions as well as a power tilt version. They not only push better but Reverse is actually effective as opposed to a regular prop outboard. Check-em out. Phil

 

Nice boat and pretty rebuild, well done and good boating to you...

 

Pardon my ignorance, but I'm really trying to picture the boat being discussed. Can someone post a pic of either the boat or something similar?

I'm from the world of planing hulls and I find 10 hp on a boat that huge interesting. Certainly I'll be doing more reading on displacement hulls.


Thanks,

RL.

 

Just go back 11 posts and follow the link. It is a displacement boat. Good for maybe 6 knots.

Rick W