How light is too light?

Discussion in 'Boat Design' started by curtis73, Jul 21, 2015.

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

    Light is likely to be a liability if the boat does not sit on its designed static waterline as a result, regardless of whether the structural integrity requirements are met.
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Light is just heavy enough to not break, with slightly more than generally expected loading. The loads at 70 - 90 MPH are tremendous.

    Curtis, except for the top speed range, your SOR is pretty typical. The crew and guest compliment is okay (1-4), though 6 to 8 is just a dream on a boat of this length, especially if you want to cook over 40 MPH. Many designs exist for a flats style of boat in this length, that given enough power will run to as much as 50 MPH, but getting over this, with this length requires a wholly differ hull shape. Flats hulls typically have low deadrise a fairly full entry to help get it up on plane quickly and other hydrodynamic tricks to help it's shoal nature. Once this type of hull gets much over say about 40 MPH, it runs into stability issues. The chines dry out, she'll drive "over center" and purpose, the ride becomes so violent that it's nearly unbearable, etc.

    Attached is one of my designs and a real 90 MPH boat. As you can see, it a wee bit longer than your desires (30') and nothing close to the configuration you'd like, but a real get up and scoot monster. 90 MPH is holy grail territory and simply difficult to do with much less than a full up racer. Doing so with 21' and 4 crew is exceedingly difficult from a design point of view. If you can live with mid 40's (which is cooking good for most), you can have a small boat, with modest draft and some accommodations capacity.


    This is the boat I'd recommend and she's 20' on deck. Though designed as an economical, light weight boat for backyard builders, she can take a lot more power and scoot along pretty good. With a 40 HP outboard, you'll break into the low 30 MPH range, with a 75 HP outboard she'll cook along enough to scare the crap out of you, without busting the bank at the fuel pump.

    Attached Files:

  3. Ike
    Joined: Apr 2006
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    Ike Senior Member

    I think your carriage and speed requirements are incompatible. You want to carry 8 people in what is essentially a bass boat with low freeboard and is 19-21 feet. Maybe if you raise the freeboard providing more volume and more reserve buoyancy.

    Getting it to go 70-80 mph is going to take a lot of HP and and a lot o gas. Gas weighs a lot (about 6 lbs per gallon) The more gas the heavier your boat will be and the larger the tank is requiring more structure to support the tank when the gas is sloshing around while you are cruising through that 2 foot chop.

    Recommendation: raise the freeboard, more people, more gear. Lower the HP. Keep a cruise speed of 25-40 but reduce max speed to about 50. Less HP less weight, less gas less weight, Less requirement for flotation in case of swamping (also less weight), longer cruising range, lower structural strength requirements due to less stress on the boat. And so on...

    And frankly if I were cruising out there in the wilderness I would want two engines. Yes that's twice the cost and more maintenance, but you can buy two smaller engines, and should one fail, still get home.
  4. curtis73
    Joined: Mar 2002
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    curtis73 Junior Member

    I appreciate the responses, but I need to clarify a couple things. I don't need 80mph with 4 people, and certainly not with 8. I don't want to carry 8 people, but I want the physical capacity to if the need arises. I want the physical capacity to not sink with 8 people, and a USCG plate that says 8 people. Not comfy, not fast, but I have a massive family with about 15 boats on this lake. If we're all out on an island for a shore lunch and my boat is heading in, I need to be ready to potentially carry as many people as I can. I don't even care if it planes with 8 people.

    or to put it in car terms since I'm a car guy; if I have a performance sedan that pulls 0.86g on a skidpad and does 0-60 in 5 seconds, that doesn't mean I expect it to perform the same way with my family and our luggage in it... but I do expect it to be able to physically still function as a car with my family and luggage. Does that make sense?

    For instance, my Baja 19SS; with just me and 350 hp from a Vortec 350, 60 mph all day. Boat was rated for 7 people max. So I don't think its out of the question to ask for 70-80 with a capacity for 8 in a 19-21' boat with just me in it... or is it?

    I'm not saying I want all of the above at the same time. I want a boat that can do 70+. Normal personage will be 1-4, but I realize that I won't be doing 70+ with 4 people.

    And the 8 number is kind of arbitrary. Initial design idea has seating for 7. Its just that the SOR asked for how many people. I should have clarified that I was stating max people, not how many people would be using it daily.

    My performance goals in my SOR are assuming 1 person.

  5. curtis73
    Joined: Mar 2002
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    curtis73 Junior Member

    My experience with flats-type hulls agrees. You put it more eloquently and technically, but I agree. I need enough deadrise to keep me planted at higher speeds. Truth be told, I was mostly happy with the hull on the Baja. It liked to wander a lot at slow speeds which was frustrating, but at 60 mph I was very confident and it was very stable, even in modest chop... mostly due to its excessive weight. I kept having this feeling that if I could ditch 500 lbs and shallow up the vee a little it would have been perfect (at the expense of still wandering at slow speeds)

    My initial thought for this build was to take that basic hull style, but build it with lighter construction and a maybe a couple fewer degrees rise (at least toward the stern) with two goals in mind; 1) reduce draft, and 2) help with top speed potential.

    Wish I could. If I'm investing the time, energy, and education in doing this for "just" 40mph, I would just buy another cookie cutter I/O and be done :) If I just wanted 50 mph, I would get a fish 'n ski. But take a look at my last post... I don't need it to perform like this with 4 crew. When I think of boat's peak performance, it always assumes one person and only essential gear. Think of this boat as a BMW M5. When I'm alone, I'll blast through the gearbox with my foot to the floor, but it can also be used for 4 people and their luggage on a nice comfy road trip.
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