Two-Plus - How Small Of A Motor To Plane???

Designs for inboard or outboard power

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tpelle
Posts: 24
Joined: Mon Nov 12, 2007 8:34 am
Location: Northern Kentucky along Ohio River

Two-Plus - How Small Of A Motor To Plane???

Postby tpelle » Thu Nov 22, 2007 6:55 am

I'm interested in building a trailerable plywood-on-frame cabin cruiser that would get reasonable performance (above displacement hull speed!) on a low-horsepower outboard motor. I'm thinking about 25hp to 35hp.

Initially I was quite taken with the Rabl Puffin (which promises adequate performance on 15hp!), but find that the plans no longer seem to be available. I do have Sam Rabl's book, but don't think I'm capable of building it from the drawings in the back of the book.

However, I notice that all of the boats that seem to originate back in the '50's and '60's promise adequate performance on low horsepower - probably because that was all that the outboards of the day would deliver, I suppose. Also, I notice that they seem to have as a common feature a rather narrow transom (as opposed to "modern" designs where the full width of the beam seems to be carried right back to the transom).

As an alternative to the Puffin, I found the Glen-L Two-Plus. It seems to have all of the features that I want, and it's hull seems to have all of that narrow-transom goodness of the Puffin.

I notice that the specs allow for outboards of up to 135 hp! I'm guessing that these are "2-stroke horsepowers", and that a modern 4-stroke motor would be heavier at a given horsepower, but I'm not interested in how big of a motor it will carry, but instead, how small of a motor will deliver adequate performance.

For me, adequate performance means being able to get up on plane, with maybe a little to spare, with two souls-on-board. I would plan on building fairly light, not adding a buch of "stuff" (refrigeration, TV, Radar - just keep it to the bar minimums) and would not plan on towing water skiers or any towable water toys - just cruising.

The area of operation would be on the Ohio River, in the Markland and Meldahl pools - which would cover the region from, say, Louisville, KY to Marietta, OH. Currents during the boating season run from 0.5 to 1.5 mph, and one would need a sufficient turn of speed to be able to maneuver out of the way of the big tows - as well as to avoid the jet-ski's and cigarette boats!

Tohatsu publishes fuel consumption figures for their outboards on the web, and a 30hp 4-stroke is spec'ed at 2.6 gallons per hour. If one throttles back to cruise RPM, maybe get to the vicinity of 2 gallons per hour at 20 mph, that would give a fuel economy of around 10 miles per gallon. (Not gonna compete with my wife's Hyundai, but at least it's not rated in dollars per minute!) This might be a cruising machine!

So, back to my original question: How small of a motor - modern 4-stroke outboard - would be needed to power a Two-Plus?

upspirate

Postby upspirate » Thu Nov 22, 2007 8:21 am

Looking at the other customers Two Plus,I would deduce that the one from Norway using the 60hp 4 stroke Yamaha,provides the performance you desire.Not sure if you would get the fuel economy,but this is a good size boat.
Just my 2 cents

tpelle
Posts: 24
Joined: Mon Nov 12, 2007 8:34 am
Location: Northern Kentucky along Ohio River

Postby tpelle » Thu Nov 22, 2007 6:07 pm

upspirate wrote:Looking at the other customers Two Plus,I would deduce that the one from Norway using the 60hp 4 stroke Yamaha,provides the performance you desire.Not sure if you would get the fuel economy,but this is a good size boat.
Just my 2 cents


Hmmm......Tohatsu offers a 50hp and a 70 hp TLDI (which are 2-strokes that meets the new emissions standards) - the 50 burns 5 gph and the 70 6.4 gph. I kind of like Tohatsu - they are the builders of both Nissan and one other big name brand of outboards (I forget which one), and they seem to have a good rep with commercial users - since they publish complete specs and manuals on the web. I spent some time in Portland and Astoria, OR last year, and I saw quite a few Tohatsu's in use by commercial fishermen.

upspirate

Postby upspirate » Thu Nov 22, 2007 6:44 pm

I've owned two Tohatsu 18's & love them.I've thought about putting a70 or 90 TLDI on my Searay Laguna 16,but don't think I want to re rig from my current Mercury 75.Will probably go with a new Mercury 75 or 90 Merc Optimax so I can use the same controls,bolt holes etc.
But it sounds like the 70 TLDI would be a great choice for your application & with the FI will be fuel efficient.That's the one I'd use


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