My 32 foot powercat design for offshore fishing / day trips in the Gulf

Discussion in 'Boat Design' started by kengrome, Jan 1, 2010.

  1. sabahcat
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    sabahcat Senior Member

    My last 32 ft sailing cat had rocker, as sailing cats do.

    It squatted at full noise with a Yamaha 9.9 outboard
     
  2. masalai
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    masalai masalai

    The squatting tendency is overcome in the displacement designs of Tennant, Schoinning, Chamberlin and others - see the Chamberlin C10 in my gallery, http://www.boatdesign.net/gallery/showphoto.php/photo/9660/ppuser/22903 subtle but works... There are three key factors the flat "planning area" holding the beam waterline all the way aft - note the waterline stain in relation to the underside surface - a modest lift with minimal penalty, also the transom clears any eddys at very low forward speed. - The canoe form extends sufficiently to allow a horizontal propeller shaft of total length around 4 ft. - The rail above the waterline also does something to reduce spray and enhance the ride... The C10 ran twin 50hp diesels giving 16knots or so for a 10m (33 ft boat) and almost no wake...
     
  3. kengrome
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    kengrome Senior Member

    Did it also have a transverse flat bottom at the transom (or anywhere along the aft portion of the hull) and a transom that was the widest part of the hull? The reason I ask is because I think these features will prevent most of the squat typical of displacement hulls when they are overpowered and being pushed to go faster than displacement speeds.

    I think that all boats squat when under power. I also think longer boats squat less. I also think boats with particular hull shapes squat less. I'm hoping a hull similar to the one I've shown in this thread will squat less ... and transform that energy into lift via the hull's planing shape.
     
  4. kengrome
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    kengrome Senior Member

    Masalai, the Chamberlin boat looks like it has a Seabright-type box keel. I'm not surprised at its low power requirement or its efficiency at moderate displacement speeds because of this. But I think the box keel will prevent such a boat from planing very effectively even with large engines.

    Obviously planing at 50 knots is not what the Chamberlin boat is designed for. But mine will be designed for fast planing, and that's why I don't think I can use a box keel -- otherwise I would use one because I love the advantages of box keels.
     
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  5. masalai
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    masalai masalai

    Ken I do not know why you attribute someone's name to the shape, they do not own it?... It is DESIGNED as a displacement hull-form, to plane a boat needs the POWER to push it up ON TOP of the water as with the early Power & Shark-Cats... to plane is not elegant or efficient, a brute force response - cheap and easy whilst fuel is also cheap... There are lots of boats that plane to 50knots and those hull-forms in your post appear to be lacking that capacity too - Have a look at maratimo, also in my gallery, at 40 ft and around 4500kg and just a pair of nothing special V8's can do 150 in fairly sloppy sea conditions if you are as insane as some of the race drivers are... but be quite 'reasonable' when de-rated to about 40knots or less... - - but WHY :?: - another pissing contest? - What would justify such a ride? - - so the lady companion shits herself and never wants to see you again? - - - - Rather, to have her feeling comfortable, at ease and sipping an appropriate beverage and not alarmed by the sea-state outside because all seems so efficiently in control...

    May I suggest you charter a fast monohull and take a mixed party for a ride at 40 or so knots, in calm seas they may be well pleased but if it gets a bit rough you will be cursed, cleaning up vomit and forced to slow down to 10 to 15 knots or LESS and wallowing if the boat is unhappy at slower speeds...

    Planing cats have been called "pile drivers" for good reason, they are not equipped with soft-suspension or shock-absorbers... so they are a rough-house day trip boat that has to go when it has to go for short distances (less than 50 miles) and rescues in almost any weather...
     
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  6. Chris Ostlind

    Chris Ostlind Previous Member

    Masa has it spot-on, Ken.

    Either go for a planing design that has a flat rocker run aft and mirrored hull forms, OR go with the current take on power multihulls and go for good speed in a semi-displacement regime that yields great fuel mileage and costs oh-so-much less with their smaller engines.

    When one actually gets out on the water in what are typical afternoon, chopped-out conditions, one soon finds out that trying to blast along at 24+ knots is a real ball busting exercise that will disturb the guests, kick the crap out your boat and soon find you with an invitee list of, "Sorry, can't make it this Saturday" responses.

    A nicely designed, semi-displacement boat, as long as you propose, with modest engines and a modest speed target, will provide many thousands of hours of pleasure and it won't send your fanny to the poor house with fuel bills.

    There's more than a few reasons why the newest commercially built multihulls are not looking for planing speeds in their designs.

    Something to think about.
     
  7. kengrome
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    kengrome Senior Member

    The 50+ knots planing speed goal is only to provide the ability to go that fast. Like I said earlier, I will seldom run that fast ... but if the hull is not designed for it then it would be impossible, wouldn't it? I simply want the possibility and I think I can get it while also having a boat that runs economically at displacement (or semi-displacement speeds) too.

    Why do you say they seem to be lacking in this capacity when I posted renderings of what are essentially Seaknife hulls -- designed specifically for high speed planing? Is there something about Seaknife hulls that make you think they cannot handle high speed planing?

    I don't have to charter one because my friend has one and we have taken it out a dozen times during the past several months. It is a hard, physically demanding, and dangerous ride at speed when a few little waves or chop appear. I almost broke my teeth coming down off a 2' wave at speed one day because these wide hulls simply cannot cushion the ride at speed like a much sharper and narrower hull can.

    This is one of my motivations for designing a nice riding high speed power catamaran. I actually want my own future boat to ride comfortably in the same conditions where all these high-HP monohulls have to slow down. And I'm not suggesting that I would want to continue going fast in these same conditions all the time or even most of the time. But if I choose to go 50+ on the water I want to do it in relative comfort and safety compared with most of the other boats I've been on that can run at these speeds.

    Maybe the existing designs do what you say, but that doesn't mean mine has to.

    May I suggest that you read about Seaknife hulls then imagine a catamaran with two of these hulls? Because that is basically the concept I am proposing here.
     
  8. kengrome
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    kengrome Senior Member

    I understand what you're saying Chris, and if I were forced to make a choice between a semi-displacement catamaran that could only run at displacement speeds or a catamaran designed for high speed planing I would take the displacement speed boat hands down.

    But after seeing the performance characteristics of Seaknife hulls on successful high speed rough water planing craft, and noting their fine entries and narrow hullforms which are consistent with efficient displacement speed boats, it occurred to me that I might just be able to design a boat -- specifically a power catamaran -- that could plane at high speeds when it has to, yet the very same boat could also be economically driven at displacement speeds and possibly semi-displacement speeds all the rest of the time.

    If my final design achieves its design goal this is never likely to happen, because even if I run the boat at high speeds in a chop it will ride nicely enough that the passengers will want to come back for more.

    I totally agree, and I may end up settling for this type of boat. But just imagine if the very same boat could hit 50 knots in a few seconds whenever you needed or wanted it to -- and do it safely without injuring the captain or crew. Wouldn't this be a great advantage to have?
     
  9. sabahcat
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    sabahcat Senior Member

    Nice bit of footage in the "Top Gear Ferarri Daytona Vs XSR48 powerboat "race

    http://www.youtube.com/watch?v=DZgV3WyEMvE

    Very brutal ride in fairly flat conditions
     
  10. Guest625101138

    Guest625101138 Previous Member

    Ken
    You do not need a wide transom to plane. Why not keep the canoe stern all the way aft. Place the outboard down low so the prop stays in the water as it lifts.

    Here is a good example of canoe stern that planes:
    http://www.woodenboat-digital.com/woodenboat/20060506/?pg=73

    I know from my own experience, and now methematically verified, that you can get nice lift with a slender hull having a canoe stern once you get some speed up:
    http://www.boatdesign.net/forums/boat-design/predicted-squat-drag-6m-hull-30831.html

    If there is any rocker it will be slight for a design speed of 20kts and mostly in the nose to give some initial lift.

    Maybe you can get Leo to do a hull design based on it being optimised for 20kts and 50kts using Flotilla.

    One of the problems in going for a dual speed boat with such a high top speed is that you are carrying all the extra strength and power all of the time for infrequent use. A bit like having a V8 in a Vespa to go shopping through the week and then compete in the annually in a drag race. It might look vaguely like a Vespa but by necessity of carrying that motor and being able to handle the power it would have the weight of a truck.

    Rick W
     
  11. Chris Ostlind

    Chris Ostlind Previous Member

    Ken,

    I wish it weren't so, but there's just no free lunch on this problem. High wetted surface, which is what a typical planing hull is much more about when it's not planing, is not going to be kind to you at slow speeds when it comes to slogging through with a hull not appropriate for the task.

    Conversely, a semi-displacement hull form is going to take massive amounts of engine noise to take it to the speeds you suggest... and it probably won't get there, anyway.

    You simply have to pick your preferences and dial it all in to that area.

    Best of luck with this,
     
  12. kengrome
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    kengrome Senior Member

    Hi erik, thanks for the contribution, here is my reply:

    If you're correct it means the hulls can have straight flat bottoms ... which means construction will be even easier than I had anticipated for the hulls I showed at the top of this thread.

    Did you also make the bottoms triangular? How about the hull sides, are they vertical or flared out a bit? Just checking so I can understand the parameters you used ... and thanks for doing these calculations! :)

    I didn't think much of the lift would come from planing at such a large L/B ratio at my target crusising speed range of 20-25 knots. I was guessing that higher speeds would be necessary before planing lift becomes dominant. Thanks for confirming this.

    You're probably right.

    It will probably require building the boat before we can figure this out, but I expect the drag to be low given the narrow transom width so it probably doesn't matter all that much in the grand scheme of things.

    Wow, the difference here is more than I expected ... but the big difference is mostly because 7 knots takes only 1kW. I can understand 13 kW for 20 knots, but is it correct when you say only 1kW for 7 knots?

    No, of course not!!! :)

    I have no idea about this.

    Rick did a GODZILLA run for me on a 9m hull optimized for 22 knots. The result without any hullform constraints is basically a simple half-circle cross section over most of the hull length with the ends narrowing down to points. Obviously it won't plane because it doesn't have anything even remotely approaching a flat bottom or a flat cut-off transom, so I may ask him to do another run with a hull that looks more like the one shown above.

    But apparently Rick has a new HPB hull that is easily built using hard chines and flat panels and it is one of his most efficient to date. The simple fact that he is getting so much efficiency out of a hard chined hull suggests that long skinny planing hulls may be almost as efficient at displacement speeds as the round bottom hulls we typically think of as being the most efficient at these speeds.

    This also encourages me to stick with my current hard chined hull design (perhaps without the rocker) because if Rick's experience translates to larger and higher speed hulls it means I'm on the right track to creating a hull that is not only very efficient at low speeds but that can also plane "nicely" in a chop.
     
  13. kengrome
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    kengrome Senior Member

    I guess it's because I've been "stuck" on my goal to keep the bottom and the transom as close to the shape of a Seaknife hull as possible -- because this hull's performance is stable and predictable at high speeds. I understand that if I go with a canoe stern I can plane, but I'm pretty sure that a change of this magnitude will impart questionable handling characteristics to the hull if I dare to drive it anywhere near 50 knots.

    So I think I should stick with the Seaknife hullform and give myself the 50 knot top speed I've been talking about ... or else just forget about this top speed and optimize for a slower top speed with a displacement or semi-planing type hull.

    That's basically what I tried to show in my hull images in this thread, although it may not look that way at first glance because there seems to be a 2-3 degree downward pitch in those profile images.

    I don't really know him but I guess I can ask. It would certainly be interesting to see what he comes up with, especially if he can explain the results in layman's terms ... :)

    If there is any reason why I might want to abandon this concept this is it.
     
  14. Guest625101138

    Guest625101138 Previous Member

    Ken
    You might find this paper interesting:
    http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=8895319&article=0&lang=en
    Particularly figures 10 and 15. The results show similarity to what Leo produced for my V14 hull although they are very different shapes.

    A wide hull works efficiently for planing in the same way a wide wing is more efficient than a narrow wing. However one interesting aspect of a transom is that it is actually pulling the boat down rather than lifting it. So bow-up trim is a combination of bow lifting and stern sinking. Notice the pressure in figure 10 at the stern is negative. Also notice this is the result of the water moving backwards as it tries to escape to atmosphere from under the hull - figure 15.

    A canoe stern has reduced area to work on so the force and lever arm acting downwards is reduced. A long hull with a narrow stern will not change trim as much as a wide hull with broad transom as the dynamic lift force increases. The WHIO really demonstrates this well.

    I am doubtful if a full width transom on a long slender hull offers any benefit from an efficiency perspective. Certainly in displacement mode it is a distinct disadvantage. Setting rocker to get it to stay clear is beneficial only at very low speed. A canoe stern is just as good at higher speed in my view. It does not preclude planing.

    One disadvantage of the canoe stern is that the boat will not turn as easily as you can achieve with a broader stern that can generate lift when going sideways. It will still turn but more like it is on rails. It will probably lean out on a curve rather than lean in. This could be nasty at 50kts but quite acceptable at 20kts.

    Rick W
     

  15. masalai
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    masalai masalai

    sabahcat,
    I'd have gone for a maratimo.... far more Chutzpah :!: and a slightly better ride... and would have outpaced even helicopter pursuit... :eek: :eek: http://www.boatdesign.net/gallery/showphoto.php/photo/15578/ppuser/22903

    Ken,
    The question still remains... Why so fast? most motor passages in something around 40 ft will find 12 to 15 knots as a reasonable maximum for comfort because of the waves in an ocean or the chop closer inshore... and the most comfortable ride is in a high bridgedeck clearance displacement cat with fairly skinny hulls...

    That is what my model was all about... In the tests its equivalent weight was calculated to be equivalent to 11.5 tonnes and 100kw engine pair would result in a potential max of around 25knots but need more fuel for the range I sought (2000nm) - ugly as it is it demonstrated effectively that 750mm waterline hull beam, somewhere around 700 to 850mm draft and a rather boxy shape as semicircular section did not give much advantage if it meant greater draft to carry the load (fuel and "niceities") for an LOA of 12m... 2 x 75kw engines would give a max of about 20 knots pushing 6 to 7 tonnes, including 2 tonnes of fuel at the beginning of a long delivery passage... Comfortable cruise 9 to 14 knots what more could you ask for?
     
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