Low speed planing hull theory

Discussion in 'Boat Design' started by Doh, Jan 5, 2020.

  1. Yellowjacket
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    Yellowjacket Senior Member

    The biggest problem is that you're going electric. If you were doing this with a 5 hp outboard it would be a piece of cake. The weight of the propulsion system is what is making it difficult. Small racing runabouts are about 8 feet long and weigh about 50 kg without a motor. A small motor like that is maybe 25kg. This leaves you with about half the weight of your electric approach. This means you'll be planing 75kg less if you go with an outboard and in this case that's huge.

    Small planing boats like this really respond to hook in the bottom or to the use of full width planing flaps at the transom. This makes it much easier to plane at higher bottom loading. The width of the hull needs to be around 3.3 meters (four feet) to get the bottom loading down and get over the hump.

    As noted, it's doable, but you need to be careful or you'll end up in stalled trying to get over the hump.
     
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  2. Doh
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    Doh Junior Member

    Hey! Thanks a lot :)
    I read the article and it's pretty interesting :) I don't really think I will achieve big speed to be honest my target is to use the benefits of an electric drive (lots of torque on short burst) and get some kind of semi planing. I keep this in mind thought. The resistance drop is actually exactly what I'm aiming for. Again really that k you for this :)
     
  3. Doh
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    Doh Junior Member

    This.. <3
    My first intent was to simply understanding raw physics in order to get some order of magnitude concerning my project.
    I like to get a reality check before spending hours, days hitting physics law boundaries in the face ^^
    I unfortunately need to keep the inflatable side of the boat. I know it's then a toy, not a boat. But the transportability is a must for me.
    I plan first to ride it on lake and river here in Germany. In the future, if the concept work sufficiently and with some modification maybe at the sea. But this is very very far from my initial goal.
    Concerning handling and safety. It's currently not really important for my little proof of concept. It certainly will and that will implies tons of new restrictions and modification. But then again, right now, I want to plan my little thing in "perfect" conditions only. We have good beers here if the weather isn't nice, I will wait ;)
    Thanks for the link!
     
  4. Doh
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    Doh Junior Member

  5. Doh
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    Doh Junior Member

    Thanks :)
    Actually my dinghy is 20-25kg, my motor only is 10-12kg and the battery will weight between 8 and 10kg
    This kind of setup is really light compared to equivalent ICE system... Minus the range and versatility. That said, if I get about 2h of plan, I could consider me really happy :)
    That said, I didn't check the small race boat section. That could be interesy to see, specially if they have low-power, what kind of solution they used.
    Thanks!
    H.
     
  6. portacruise
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    portacruise Senior Member

    My out-of-the-box speculation is releasing a burst of air bubbles or pockets to temporarily reduce resistance under the hull might get you up to planing speed with very low power. But I don't think it's been done with very small inflatable boats and it may be too complex or unworkable for your situation.
     
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  7. clmanges
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    clmanges Senior Member

    Is there any reason to believe that approach works? It seems to me that introducing air into the water would reduce that water's buoyancy.
     
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  8. Jimboat
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    Jimboat Senior Member

    Clmanges - I agree 100%
     
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  9. fallguy
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    fallguy Senior Member

    This is a very confusing thread.

    The idea of the least most planing hull that will plane if the operator diets with multiple propulsion sources is nearly an oxymoron; not to mention bordering on silly.

    The SOR is incomplete and plugged with solutions.

    Complete the statement of requirements without unneeded caveats like dual power, etc. I don't really understand intended use well. Most power restrictions are speed restrictions and the 2.5 meter restriction is typically a beam restriction. An 8 foot dinghy can only be a monohull, so where did the idea of a cat come from?

    Corrections welcome.
     
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  10. Doh
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    Doh Junior Member

    Hey,
    The 2,5m and 3,6kW restriction are simply french legislation. Anything under this can be used in an 300m radius from coast.
    I don't really know where the dual power idea comes. I will have only one source. But I may missed the sense of your sentence
    There is some small cat with 8" with cat to semi cat design.
    That said, it seems that the surface in contact with water is an important factor to the planing equation. I was actually wondering if a cat oriented dinghy would give something like a mix of both. I've seen people talking about the takacat and be pretty happy with them. There was specially some statement about consumption benefits, but it seems that they are more on the top speed area, which is in my case irrelevant as I do not thing that my little thing will achieve anything which could be called "high speed" ^^
     
  11. fallguy
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    fallguy Senior Member

    You'll have to show me those regulations; they seem nonsensical to me.

    On the balance, the 300 meter radius seems to be a speed rule that say 5 knots max on my read. So, you are wanting to plane in an area at under 5 knots that is essentially considered a no-wake zone. This is silly to me and not even within the spirit of the law.

    There is no rational person here that can suggest a hull length of 8 feet for two cat hulls.

    Like I said before, something here is not right, either my understanding of the SOR or your understanding of French laws or your idea of French law or French law itself perhaps, or my understanding perhaps, but I doubt the laws are designed in the fashion you are purporting. I mean you well my friend.
     
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  12. DCockey
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    DCockey Senior Member

    Why can an 8 foot boat only be a monohull?

    The Catyak was a 9' catamaran and thousands were sold. A park near us had a rental fleet of Catyaks. Vintage Dayton Catyak Sailboat http://yachtsboatslist.com/other-makes/66479-vintage-dayton-catyak-sailboat.html

    I talked to a very experienced wester US white water guide who took trips with friends on white water using Catyaks. WATER RECREATION FLOATING A NEW RIVER APPROACH NEWCOMER OPENING A NEW 'VISTA' FOR THOSE RIVER-RUNNING ENTHUSIASTS BENT ON NEW THRILL https://lmtribune.com/nation/world/water-recreation-floating-a-new-river-approach-newcomer-opening-a/article_e79933c2-22cb-5885-8ecd-59aac7d3fba6.html
     
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  13. DCockey
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    DCockey Senior Member

    Based on my understanding of planing boat design:
    For the same bottom area and shape when viewed from the top a hull with lower deadrise will have less resistance but a rougher ride. Zero deadrise will have minimum resistance.
    For the same displacement and same bottom area when viewed from the top a hull which is shorter and wider will have less resistance at higher speeds but larger hump resistance when getting onto plane.
     
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  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Like some others here, I am confused about what you want to do with this thing, to be needing planing speeds. Traversing anything but quite sheltered water is out of the question, and it seems to me your ambitions of how far and fast you want to go, are going to run into barriers caused by battery weight, unless they are quite modest duties.
     
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  15. Ad Hoc
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    Ad Hoc Naval Architect

    AS noted above by Fallguy....just define your SOR and then draw the arrangement/design you want. Then establish if this is feasible. If not, what changes, if any, can be done to make it work.

    It is that simple, not rocket science! You are over thinking this as a 'theory' exercise...it is not.
    And if you, as you currently are doing, focus on the small details, you'll miss the big picture - the design!
     
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