18ft planing boat, how much hp?

Discussion in 'Boat Design' started by Ozparker72, Apr 18, 2015.

  1. Ozparker72
    Joined: Mar 2014
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    Location: uk

    Ozparker72 Junior Member

    Thanks Mr Efficiency

    I've done some approximate calcs
    And I reckon she weighs between 3300 and 3800 lbs

    The original ford engine weighed around 380 lbs + 120 lbs for the Volvo penta leg
    So 500lbs in total.
    A 90hp 2 stroke should weigh around 300 lbs.

    Plus she was fitted out quite heavily inside, I'm stripping out a lot of un-necessary weight. Maybe losing another 300 lbs

    So total weight saving of at least 500 lbs

    I'm hoping this should compensate for some drop in HP from the original?
    Assuming that her original rating of 130 hp, was Ample, do you think I can maybe get away with losing 40hp, and still get her on the plane?
     
  2. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    Please, do not reckon, weigh the lady.......................Also weigh gas at 6 pounds per 1 US gallon and water at 8 pounds a gallon.
     
  3. yofish

    yofish Previous Member

    "A 50 HP would likely get the boat up," Wanna bet? Do you even really look at pictures full size before making statements? It does have chine flats.

    Ozparker, if you choose to follow PAR's advice you will, in no time, regret it. There is a small industry here in Alaska by doing these conversions. The fact is that most people do not keep up with the maintenance on an I/O and rot rather dramatically in damp environs. Every one of these conversions that I've done has always elicited amazement from the owners with how quieter, more fuel efficient and usually faster the boat becomes. They all closely duplicated the original horse power, never less than.

    Those boats from that period, Crestliner, Glastron, Sea Ray, Bayliner and Reinell etc., were well made and usually pretty heavy. Many of them also had a fatal attraction to early type I urethane foam for hull stiffening and floatation. This was placed in the voids from the two main longitudinal stringers/engine beds then plywood glassed over it. I can not tell you how many of these things that weigh a bazillion pounds from that foam, which is a dandy sponge, soaking up water from the decks leaking. You may find that your skiff suffers this. If so, it really has to be addressed because of the obvious reasons.

    I certainly understand your desire to keep the cost down both in initial capital investment and fuel use - BUT - alas there is this thing called 'false economy'. Should your new 70 honda puke five years from now for it being asked to work at its higher end, what's been saved? I can also absolutely state that the motor running at the top is consuming the maximum amount of fuel.

    This is hardly a shallow deadrise skiff! Believe, getting this puppy up out of the hole with a 70 would be a chore. That boundary is a big one and is also the one that puts the MOST strain on a motor. This also implicates the propellor. One can down pitch the prop in order to keep the revs up which is easier on the motor but then, when you do get on step, the motor is now turning up higher with no increase in forward thrust! Bummer!

    Say that Mr. E is correct and it did have a 130HP as original. Think about that.

    Don't listen to any of us, Do your own D.D. This forum can be a bit of a wasteland, frankly. But to help you in D.D., here is one of many that may be pertinent from the tubes:

    "For the purposes of Boating’s test, one of the 188s sported a 115 hp Evinrude E-TEC outboard, the other a 190 hp MerCruiser 4.3-liter stern-drive. Obviously, that’s not an apples-to-apples comparison, but it’s the reality of today’s market.
    The aforementioned emissions laws have pretty much done away with the availability of the once-popular 135 hp 3.0-liter stern-drive. While the days of the carbureted 4.3 may also be numbered, there are simply more of them in inventory, ensuring their availability in the near future.
    Despite what seemed like an obvious disadvantage on paper, the outboard proved a worthy competitor, part of which can be attributed to its significantly better power-to-weight ratio. Coupled with an Alpha drive, the MerCruiser 4.3 tips the scales at 865 pounds; by comparison, the Evinrude 115 weighs only 390.
    Coming onto plane, the outboard came up and over the hump in 3.6 seconds, the stern-drive in 4.8. While the stern-drive’s greater torque likely helped it post the better time to 30 mph (seven seconds), the outboard still kept things close, hitting the mark less than one second later. The stern-drive’s added horsepower truly resulted in a clear advantage only at top speed. The 115 hp outboard peaked at 43 mph, while the 190 hp stern-drive continued on to finish just a fraction shy of 48 mph (47.9).
    Torque is worth considering when you compare the two forms of power. It’s directly proportional to engine displacement, meaning the 4.3-liter stern-drive has a decided advantage. Torque has practical benefits, like allowing the boat to stay on plane at lower speeds or muscling a skier out of the water in a deepwater start.
    Outboards are presumed to be more fuel-efficient at low speeds and stern-drives more efficient on the top end. Our test proved half that statement. Looking at the fuel-flow data of our two test boats (as well as results of several additional tests with similar engines), the outboard burned 30 percent less fuel at 5 mph. A big percentage, but a small difference in actual fuel burned. If you idle often, it results in savings. At 31 mph, the outboard consumed 7 gph, while the stern-drive burned 7.6 gph at 32 mph — nearly a dead heat. At its peak 43 mph, the outboard burned 11.1 gph for 3.79 mpg compared with the 48 mph, 2.77 mpg stern-drive’s 17 gph."

    http://www.boatingmag.com/boats/outboard-advantages
     

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  4. whitepointer23

    whitepointer23 Previous Member

    If I was you I would see if there are any good used v p engines around. The 130 hp twin carb b20 is a terrific engine. Much quieter and more fuel efficient than old 70 2 stroke. The b2o hangs off the transom assembly so you can bolt it straight in with no structural work at all. I have had several boats in that size outboard and volvo pentas . I would have a b20 before an outboard any time.. thats my 10 cents worth.
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You certainly would not go out looking for a 70hp outboard to power that boat, however if he has been offered one at a knock-down price, you might investigate further, what can be done to improve the situation. For starters, if it is the old OMC 3 cylinder motor, which spins a fairly good size prop with a higher-than-average reduction ratio, there may be more hope than a motor with a smaller diameter prop with less reduction. I'd consider anything resembling the latter, not worth trying. Check the gear ratio of whatever motor has been offered. With the right prop (lower pitch than standard), you could likely pop it on plane reliably, and cruise at 20mph at around 4500 rpm. Would the boat run happily at 20mph without falling off the plane, running with a chop ? If not, the 70 is probably not going to do it. But I would say the odds are it will run cleanly enough, unless you pile too much weight in. Or are excessively heavy to start with.
     
  6. whitepointer23

    whitepointer23 Previous Member

    Where did he say he had a honda. You're jumping up and down without reading the thread first. He said 70 2 stroke ....... by the way I ran a 50 merc redband on an 18 ft glass deep v half cab for a few weeks while my 85 suzuki was getting repaired. It planed easy and ran about 20 knots. It did use more fuel due to working harder but it did the job fine. A 70 works ok on boats that size. 90 to 115 is optimal. I would stick with volvo's myself.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yeah, I'm not sure what Yo is talking about, but the basic calculations are simple enough to get in the ballpark and 70 HP easily gets it done, even with this era build type. His point about the soaked foam is a consideration. I've pulled foam out that added hundreds of pounds of weight, burdening the propulsion considerably. It's only an 18' boat and his interests are relatively modest and though I'd consider a bigger engine, the opportunity for a good, cheap 70 is hard to pass by, especially if he puts her on a diet, hacking away at unnecessary cabinetry and such.

    Guessing just doesn't get you close enough, so get her weighed as best as you can and then fairly accurate power requirements can be made.
     
  8. yofish

    yofish Previous Member

    PAR, you really have no experience with this situation do you?
     
  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    As it was, with the sterndrive ? That is hefty for an 18 foot planing boat. The typical 5.5 metre (18')- glass boat in this country, sans engines, would be around 800-850 kg (1800 lbs-ish). But they would not typically be 8 feet wide, either. Incidentally, I like the look of the hull, and even that aircraft-carrier style bow ! :D
     
  10. Ozparker72
    Joined: Mar 2014
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    Ozparker72 Junior Member

    Hi Yofish
    Thanks for the info, that's helpful. It's good to know about the fuel saving at low speeds, I live on the water so use boats every day of the year. I'm moored on a river with a 6 knot speed limit, so will only be planing when I'm at sea (6 miles away)

    I've been offered is a 70hp Mercury 2 stroke, for £1000 (approx $1500 USD).
    I was doubtful that it has enough power for this boat.

    Mr Efficiency, I am not in a position to weigh the boat. It would be too costly in time and money. My calcs on weight are based on it being fully fitted out, with 2 people aboard.
    It is a calculation rather than a guess, I'm pretty accurate (obsessive) with calcs, so it will be "on the money"

    This doubt has been re-enforced by what you and others have told me.

    I will wait until a 90 or 100 2 stroke comes along cheap.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    If that is the old 70hp merc, yep, I'd stay out of it.
     
  12. whitepointer23

    whitepointer23 Previous Member

    why don't you look at putting a Volvo back in . it will be a better boat for it , especially with low speed limits.
     
  13. Ozparker72
    Joined: Mar 2014
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    Location: uk

    Ozparker72 Junior Member

    Hi Brendan
    The leg is a Volvo, but the old engine (ford Essex) has been stripped out.

    The leg is in poor condition and needs re-furnishing, then there's the cost of buying a diesel inboard etc,
    I fear the cost could run into £1000's that I don't have.

    I could bye an old 2stroke outboard for much less money, I can fabricate an outboard bracket in a day

    Then again, when I look at fuel consumption for outboards, I am starting to wonder if it's false economy?
     
  14. whitepointer23

    whitepointer23 Previous Member

    here we can pick up complete used sterndrives quite cheap but I don't know what the market is like in your area. the 4 cyl Volvo's are a good setup and very light to.these are petrol motors not diesel. 2 strokes are good but like to run between 1/2 and 3/4 throttle in the old models. the Volvo will be happy at 6 knots all day and still be able to head offshore at 25 to 28 knts in your boat. yofish makes a good point but the outboard engines he is thinking of cost a lot of money.
     

  15. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Depends how much use you intend to put it to. If you are going to do a lot of hours, the fuel issue is more important. I don't see any kind of diesel being viable in a boat of that type, at that size. A mid-range 2 stroke (90-140 hp ) is not onerous in fuel costs, and will be up to the job.
     
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