Outboard for a displacement hull

[Allan]

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.

[TeddyDiver]

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

[Allan]

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

[charmc]

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!

[openboater]

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.

[Frosty]

Maybe 2x5HP ??

[Guest625101138]

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.

[TeddyDiver]

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) :D .

[Allan]

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.

[TeddyDiver]

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.

[TeddyDiver]

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:D

[pfridays]

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

[masalai]

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

[RockLaker]

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.

[Guest625101138]

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

[Knut Sand]

This is also pretty much why I keep asking for dimensions for a "bigfoot" engine...

You have one here:
http://www.hinterlandmarine.com.au/outboards/101_Mariner_99_BigFoot_4-Stroke.php

They are superb for driving heavier hulls, geared differently (making the gearbox a little bigger, but at deplacement speeds, a thing like that doesn't matter that much). Handles bigger propellers with more ease than the standard version of the same engine.

Also; the chock to the gear, hammering it in reverse is something it is better designed for, as putting a heavy propeller on a standard gearbox may cause more stress than the gearbox likes to handle.

[Knut Sand]

I'm from the world of planing hulls and I find 10 hp on a boat that huge interesting.

Ok, then the first thing you will have to realize: Deplacement speed is slow. dead slow, and many find it dull.

But the engines needed is smaller, cheaper, and the money spent for fuel for time on the water is way less than for a boat with a planing speed, running on planing speed. It'll help saving our fuel reserves if more deplacement boats were built. But the theme of our time, is bigger, better, faster, more...

Also, to remember, when your back or reached your destination, the journey is over, on a deplacement boat, the journey takes longer... Just a question of altering your mental settings....:D

Look like a nice boat, what aboat just putting the engine on the transom, instead of building a well?

[RockLaker]

Whoops, never noticed the link for some reason. Read all the posts though. Thanks.

Sometimes I like to go fast, but I've always loved the idea of trawler-type cruising on a bigger boat for a long time. A small, dependable motor running for hours on end while putting along in relaxed comfort really turns my crank. It's almost like I was an offshore fisherman in a past life. ;)

Now that I've seen the boat and realize what will be done with it, I'd rather the smallest motor that would push the boat at displacement speed at about 1/3 to 1/2 throttle. Running a 10 at full throttle would be far louder, harder on fuel, and wearing out quicker than a 20 at 1/2 throttle. So I guess if 4hp is needed, as mentioned, then I think I'd go for a 15 and give it a life of light-duty work, but will run fast enough to keep the carb clean. Then make that motor a Honda 4-stroke and it'll last 100 years.



Allan, great job on the boat!





LC.

[Allan]

I've been busy most of this summer trying to finish this boat and it looks like I'll get it into the water by the end of this month at least for a trial run. We are already hitting freezing temp. in the evening here. I ended up getting a demo Honda 15hp long shaft with electric start and remote control for the price of a similar 9.9hp. I'm putting a high thrust prop on this motor as well and have installed a 20 gal. fuel tank below the floor as well as carrying a 10 gal. portable tank so I'm hoping for a range of several hundred miles which should take me most places I'm interested in exploring. I'll post some pictures of the finished boat at a later date. Actually had a chance of going on a high speed cat this summer and must admit it was the most unenjoyable means of travel on the water I've ever experienced, give me displacement hull speed anytime.

[Guest625101138]

I am looking forward to the photos and feedback from first trip.

I would be surprised if you do not find the Honda more than adequate. Electric start and remote control should give it a big boat feel. Nothing more dinghy like than having to run aft to pull a string to get the motor going.

Rick W

[Riverrat1969]

Allan, that's a nice looking conversion! I am pushing a 24 foot Carolina Skiff with a 9.9 HP four stroke Evinrude (Suzuki in disguise). I got a wider blade, shallow pitch prop, and with six people, two large dogs, and lots of gear, we do 3 statute miles per hour (GPS) at fast idle.........top end 8-1/2 MPH.

I used this Evinrude, because I already had it, but if this outboard ever wears out, I would get the 9.9 HP four stroke Yamaha high thrust outboard to replace it.

Yamaha came out with that engine many years ago, to push 25-30 foot sailboats.........way before most manufacturers started offering four stroke outboards. [COLOR="Red"]Tried and proven for a long time, the Yamaha high thrust has lower gears and a larger prop, than a typical outboard.[COLOR="red

[Allan]

Finally got the boat finished enough to try it out this weekend. The Honda outboard pushes the boat along at a nice leisurely 5 to 6 mph at half throttle and is so quiet that you can have a conversation without having to raise your voice. There is still some work to finish on the boat but nothing major. It should work out quite well as camping boat for us and on the lakes and large rivers here as we can sleep on the v-berth up forward and close off the cockpit area with the canopy when the weather gets too wet Plus there is lots of storage space onboard for gear.
You can see pictures of the boat at
http://www.watsonriverdesign.com/Boat04.htm

[Guest625101138]

Allan
Great to see you are finally on the water. The job looks good.

The modern motor is a bit out of place in an old style displacement boat but I bet there are many people wishing they could get your fuel economy. I think we will see a lot more leisure boats like this in the future. It is time on the water that counts not the speed you travel. Have you any idea what fuel economy you are achieving?

I imaging the boat enjoys the reduction in weight. I expect will be a touch more tender now but more than adequate for river and lake use.

Have you had a chance to open the throttle to see what top speed is. No doubt a lot of wave making at full throttle.

Would be nice to see a photo of the trim under way. Is there any risk of the prop grabbing air when cresting a decent wave?

Rick W

[Allan]

When I took the boat out for it's trial run I just used a day tank hooked up to the engine. I'm now in the process of hooking up the the below deck fuel tank that is located in the bow of the boat and I'm now starting to wonder if the fuel pump on the Honda 15hp will be adequate to draw fuel from the tank to the motor. I asked the local dealer and they couldn't give me an answer. The tank is located 15 ft away and 2 ft below the motor.

[Guest625101138]

I would be surprised and very disappointed with the Honda if it could not get fuel easily under those conditions. If you were using extremely thin tubing the distance may make a difference but for something with 1/4" ID or a little larger of typical fuel hose it would cause little resistance.

Rick W.