6000 # sailboat, want hull speed at ~1/2 throttle... 40 hp vintage Big twin

[dem45133]

Doing twin install on a 27 ft sailboat but twins are for redundancy... Anyone know how to calculate what pitch prop will give me a 6 knot hull speed at 1/2 throttle on either one of a '59 35 hp or a '62 40hp Evinrudes?

PS, it had a little 12 hp Chysler 280 Sailor and reportedly would provide hull speed, but at wide open throttle... I want it a 1/2 throttle so the continuous operation for long distances, thus the 40 hp... (when the winds die or are not from the right direction) will not drive me crazy.

Also... since these motors will never be operated at full throttle... what changes to the carb settings and timing should be done to maximize its output/economy in the 2500 rpm range without over doing the slower flowing cooling system? Richer oil mix to offset less flow volume through the crankcase for long term operation?

I know... I'm a bit outside-the-box....

Thanks, Dave

 

[CATransplant]

Re: 6000 # sailboat, want hull speed at ~1/2 throttle... 40 hp vintage Big twin

If I understand you correctly, you're going to mount one 35 hp and one 40 hp on your sailboat for backup power.

The prop isn't going to matter a whole great deal, and you don't have that many choices for those engines. I'd recommend the 10 X 13 props for both of them, since that's the highest pitch that's readily available. Could be 10 1/2 x 13, too, depending on the prop's source.

You'll be adding over 300 lbs of weight with those two engines, if that's any consideration.

Further, even operating at part throttle both of those engines are quite the fuel hogs. Both run on a 24:1 fuelil mix, and they suck fuel pretty handily.

You'll probably be running those at about 1/3 throttle to get your displacement speed. At that rate, running one will use about 2-2.5 gallons per hour of operation.

If your old Chrysler 12hp would attain displacement speed at full throttle, I'm thinking that you'd do a lot better with a nice 25hp engine, preferably a four stroke. That will probably run about half throttle and will use a lot less fuel than your larger engines. If you have a later model one, you can, no doubt, get away with just the single engine, as long as you maintain it properly.

I understand your redundancy issue, but...

 

[dem45133]

Re: 6000 # sailboat, want hull speed at ~1/2 throttle... 40 hp vintage Big twin

Hummm... 2-2.5 gallon per hour....

I weighed one of these, its about 90-95 lbs. I figure about 215 lbs for both with the added mounting structure(s).

Part of the issue is that I already have these engines, as well as a complete parts engine (actually it will run fine too, but is a short shaft) and they are in good shape.. I do not have much extra play money. Buying late model engine(s) is just not possible. Its will be hard this year even to justify the 7 mpg to haul it to the closest Great Lake (about 180 miles). Its lives here on a trailer I built for it last year.

Any way one can run a richer oil mix but lean out the carb for better economy for this application and power setting?. These old carbs are fully adjustable on the needle valves.

Oh, the other benefit of twins is better control and handling in tight quartered marinas. Especially the extra reverse thrust when needed.

 

[CATransplant]

Re: 6000 # sailboat, want hull speed at ~1/2 throttle... 40 hp vintage Big twin

Better weigh those again. They weigh just over 150 lb. Each.

There's no way to lean those carburetors down without risk to the engine from running too lean. The fuelil ratio, also, has nothing to do with fuel economy.

Those engines were designed back in the days of 25 cent per gallon gasoline, and power was more important than fuel economy.

On another note, most outboard engines suffer nothing at all by being run wide open. Two-stroke engines just have pistons, rods, and crankshaft as moving parts. On normal planing hulls, they achieve their best fuel economy by being run at between 2/3 and 3/4 throttle, after putting the boat on plane.

In your application, you'll pretty much be limited to displacement speed, although you could probably plow along at a couple of knots faster if you ran the engines wide open.

Since you'll probably hit displacement speed at under half throttle or so on a single engine, you won't be using the engine at its most efficient throttle setting, I'm afraid. So, your fuel consumption rate will be more than you'd expect.

Another option for you might be to sell both of those outboards and use the proceeds to buy something more appropriate for your use.

Frankly, were it me, with a 27' sailboat, I'd be considering a small, diesel inboard for an auxiliary engine. But, that's just me.

 

[dem45133]

Re: 6000 # sailboat, want hull speed at ~1/2 throttle... 40 hp vintage Big twin

Thanks CATransport...

150 lbs huh? Guess my bathroom scale is off. I just lifted one up and stepped on the scale.. said 260... less my 170 = 90, but its certainly possible it a garbage reading. maybe it only reads to 260... who knows. Didn't know I could still pick up a 150 that easy. maybe I'll keep the 12 on it and mount up a single 35 as backup... but that means differing fuel deliver systems... will have to think about that... plus the 12 has lower unit bearing issues...

Yes, I would dearly love to find a small diesel inboard... but again thats money I do not have... I have both a Volvo Penta and Perkins diesel searches running on ebay all the time. There was a single cylinder with trans and shaft on ebay once... but it was in CA and sold for better than a grand, plus it would cost close to that to ship it... its also needed a full rebuild as it was propbubly seized. I would have had better than 3k into it at a minimum... likely closer to 4k by the time it was done.

Its da_n tough to do anything in Marine or Aviation without significant sums of money. I know that people drop 10 or 50 k just into power or 100s of k even 7 figures into the boat... we simply can't and yet between us we have 5 college degrees... but it means nothing as the majority of those that can afford 6 figure plus boats did so on the backs of people like me because I won't lie or be unethical or play politics ... which seems to be the modern capitalist way anymore... we best not go here!!!!!.

Well I already have these engines so I'll set them up. I will not be doing anything to the hull itself except adding internal bracing to transfer the engines weight and thrust moments away from the stern bulkhead. The forces will only pass through it using through bolts but internal structures will carry those forces... not the stern.

It'll do for now... and when I can find a better means it will not be hard to pull and do up something else. Actually down the road I really want to find an inboard 31 or 33' 9 or 10ft beam. But thats some other year and a bigger truck.

It truely would be a lot easier If I actually had any real play money... I don't. Basically I'm re-powering for less than $150 in materials and parts using 2 engines I already had...plus a $200 40 hp I just bought.

So far here the costs:
The boat lives at home in my shed... (no long term marina fees, launch or overnight fees only)

$800 for a 27 ft sleep 6 water winnebago... fully sailable and complete, with radio and depth finder.. all preservers, lines, extinguishers, bumpers, etc.... a 1978 Bayliner Buccaneer to be exact... like I said a "water winnebago".

$1500 for materials (including the doner trailer I modified) into a triple axle (on a slider) 32 foot trailer (w/an additional 12 ft extend-able slider tongue for launch) with a total gross capacity of 13,500 lb. (professional build... more features than a factory built and more adaptable in future years for up to is weight capacity.)
$350 for all three motors which I'll use two. I still have the 78 12 hp Chysler Sailor...

$2,600 total investment and it will do extended sails right along side of anyone. Not too bad I'd say.

 

[CATransplant]

Re: 6000 # sailboat, want hull speed at ~1/2 throttle... 40 hp vintage Big twin

Well, good luck with whatever you decide to do.