How can i put a bigger motor on a smaller boat?

Discussion in 'Boat Design' started by willfishforbeer, Jun 14, 2006.

  1. willfishforbeer
    Joined: Jun 2006
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    willfishforbeer Junior Member

    Alright i've been looking at some designs for 17-20 foot flats skiffs and most of them have a low rated HP, under 90 most of the time. Why is this? Most of the production boats use 115s - 150s. Are the production boats heavier? Stronger? what is it? How can I increase the HP of a boat without sacrificing much draft.
     
  2. twakeley
    Joined: Mar 2006
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    twakeley NAME Student

    There really isn't an easy answer. As far as I know it is illegal to go above the maximum rated value for your boat. Are you referencing suggested max horsepower or the actual maximum rated from the Coast guard? Obviously this is to keep you safe and help prevent overloading the boat. If you wanted to add a bigger motor without increasing draft you would have to reduce weight elsewhere or think about modifying the hull. Then after you modified the boat, you would probably have to get it rated again by the coast guard for a larger engine. Maybe theres a different way, but I'd just look into another boat that suited my needs better.
     
  3. willfishforbeer
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    willfishforbeer Junior Member

    Alright well heres the real question i need answered. How does the CG figure out the max HP for a boat?
     
  4. twakeley
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    twakeley NAME Student

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  5. dereksireci
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    dereksireci Senior Member


    Great post. The other thing to consider is safety and the weight of the engine. I tested a small boat one time with an early 4-stroke outboard. The added weight of the engine made it chine walk dangerously, although it ran fine with the same hp 2 stroke.

    You run the formula and install the highest rated engine hp. If the most experienced boat pilot you have wets his pants at WOT then the max hp is de-rated. If he can't handle it then the average consumer is going to get injured and sue you. There is the "quick turn test" to determine if the speed and handling are acceptable. Last consider cost. Higher hp costs more which is acceptable in an expensive boat but maybe not a skiff.
     
  6. Ike
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    Ike Senior Member

    Having spent twenty years of my life in the Office of Boating safety telling people the answer to your question I think I can still give you a definitive answer.

    Recreational boats under twenty feet in length, that are monohulls, are required to meet the federal horsepower standards. See http://www.uscgboating.org/safety/boatbuilder/index.htm

    This does not apply to catmarans, trihulls, inflatables, canoes or kayaks or sailboats.

    There are actually three formulas depending on the type of boat. For a outboard powered boat with a 20 inch high transom ( 19 or more counts as 20 inch), That has remote steering of any type (wheel, stick) the formula is 2 times centerline Length X the maximum transom width) -90. The answer can be rounded up to the nearest multiple of 5. so if the answer comes out say 102.3 the HP would be 105. The trick here is maximum transom width, NOT Beam. This includes rubrails. Manufacturers have added all kinds of things to make this wider, such as fins and flare etc.

    Flat bottom hard chine boats use a different formula, as do boats that have less than a 20 inch transom or no remote steering (tiller steering). See the link above.

    To answer why flats boats all seem to be 90 HP, because everybody copies each other. Flats boats as you know usually have a low freeboard, an uncluttered deck and are pretty wide for their length so you can stand to fish. They usually have remote steering. The hard part is the twenty inch transom height. Make the freeboard higher and the transom a little wider and you hit that magical 100 HP figure.

    Another thing that comes in to play that affects the HP indirectly is flotation. Monohull recreational boats under 20 feet must have level flotation. That is they must float level when swamped, not roll over or sink. The amount of flotation is partly determined by the weight of the outboard, because it takes a lot of foam, equally distributed on either corner of the transom to float that hunk of metal without turning the boat over. Outboard motors although in different HPs are actually usually the same engine just detuned or tuned up. For instance it used to be that every Merc from 95 to 140 was the same block and weighed pretty much the same. The Coast Guard and ABYC publish a table of weights for outboards that is used to determine the amount of flotation needed and the engines are grouped by HP range for a certain size engine. So if you went above 90 you have to provide more flotation and maybe on that size boat you just couldn't get enough in to meet the standard.

    The other considerations of course are can the boat handle that much horsepower? Years ago (back in the fifties) tests were done to determine the manueverability of boats with different type hulls depending on the size of motors. In fact in Europe that's what they use to determine horsepower. They have a standard ISO test course the manufacturer has to run the boat through and be able to turn and avoid hitting a barrier at full speed. A lot of boats in the US who use our the CG formulas cannot pass the ISO test.
     
  7. safewalrus
    Joined: Feb 2005
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    safewalrus Ancient Marriner

    hey everybody else in the world (this IS an international forum) this talk about legal sized outboards is only for our American compadre who seem to think they know a thing or two about boat engines (actually they do - quite a lot) and need to 'sex' it up with all sorts of laws the rest of us can basically do as we please but just remember, why use two where one will do? economics itself will help! So as I said these guys do know what; why it's got to be a law.......:p
     
  8. Ike
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    Ike Senior Member

    Hey, It aint just the US. Canada, Britain, France, Germany........etc; need I go on. Anyway, this is why most bass boats, go fast outboards, etc are either exactly 20 feet or 20 feet 1 ", so they can put on whatever they want! There were a lot of boats in the 70's that sudden grew a few inches when all this went into affect in the US. People always find a way to get around it. Like I said it doesn't apply to catamarans. A lot of tunnel boats qualify as cats.

    Here in the US you not only have the regulations, you also have a lot of predatory liability lawyers just salivating for someone to sue for a couple million bucks because they overpowered their boat, or did something else equally stupid. So the best defense in court against these characters is to follow the standards religiously and document everything you do.

    Of course, if your're building your own boat, for your own use, and never sell it, you can do what you damn well please.
     
  9. willfishforbeer
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    willfishforbeer Junior Member

    What is chine walking. And what are the regulations for a boat over 20ft?
     
  10. wilezcoyote
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    wilezcoyote Junior Member

    I had a 16' carlson glasstron(rated for a 125 hp) with a 200 black max 25 pitch lazor 2 prop ,it did want to chine walk until I raised the engine(cavitation plate was 2" above the bottom) and installed solid motor mounts after that no problem...boat went like hell
     

  11. Ike
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    Ike Senior Member

    First What are the regulations for boats 20 ft and up? There is no HP regulation. You can put on whatever you think the boat can handle. I would certainly take it out and test it with the engine before I decided to buy it.

    Chine walking is a point at which a boat begins to rapidly roll back and forth from one chine to the other at high speed. Not necessarily the boat's maximum potential speed but usually close to it. Some studies have been done on this and it seems the engine setup is a big factor, but also the steering setup has a large effect. Some boats have had the chine walking eliminated simply by tightening up any loose play in the steering. Some boats can power through it and become stable at a higher speed but I wouldn't want to be the one to try that. If you have chine walking with your boat first try changes to the engine setup. The height, the angle , is it dead center on the transom. Maybe you need a jack plate and some set back (moving the engine farther aft). Check the steering sytem to make sure there is no free play. ALso check the bottom of the boat in the aft part of the planing area to make sure that is is straight. Any curve, convex or concave, can cause handling problems and slow your boat. Don't just eyeball it. Use a straight edge to check. There maybe a hook in the very aft end of the planing area.
     
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