Increasing speed w/ semi-displacement hull

Discussion in 'Boat Design' started by 67-LS1, Nov 29, 2005.

  1. 67-LS1
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    67-LS1 Junior Member

    I'm considering a small fiberglass trawler with a single diesel. Grand Banks 32, CHB 32 or 34, Cheoy Lee 32, etc. These all seem to have semi-displacement hulls and depending on who wrote the add copy show cruising speeds in the 7-9 knot range. They all seem to use the 120 HP Ford Lehman diesel.

    I looked at one the other day and the owner (who probably should have kept his mouth shut) mentioned that he would leave with the tides and stop for the night when the tides changed.
    WOW. That is some slow traveling. I had not though about how you could be almost stood still in a tide.

    Now my question; Is there any way to increase the speed of a small trawler like this without a massive infusion of horsepower and fuel? I'm not looking for 20 knots, but even a 12-14 cruise would mean a huge difference in the usability of the boat, given the way I boat, which is mainly weekends in and around the SF bay and delta.

    Trim tabs or wedges? Hull extension? Advances in prop technology since these boats were built in the 1970's? Any other ideas? Am I being unrealistic?

    I saw one Grand Banks 32 (wood boat) that had been converted to a Volvo diesel with a jack shaft to a duo-prop outdrive. I left a note (2 months ago) for the owner to call me but haven't heard from him/her. Was this guy onto something? Seems weird given the big keel in front of it.

    Thanks in advance for your replies.

    Dennis
     
  2. RHough
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    RHough Retro Dude

    No :)

    If 120 HP gives 7.5 knots, 15 knots will require 480 HP

    OTOH a 7-9 knot cruising speed is faster than most of the sailboats in SF Bay. That speed doesn't really limit you very much. A Delta run is a long weekend anyway, anywhere else in the Bay is no problem. Running from South Beach to Antioch for lunch is not what trawlers do. If you want 15-20 MPH cruise, you need to look at planing hulls, not trawlers.
     
  3. cyclops
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    cyclops Senior Member

    I am always confused at what a difficult selection it is to find a boat that does 15 to 18 +- a couple. What are the space requirements? food duration? clothing days? Bathing or shower? playtoys in what # and type? Miles non stop? Fans or AC? # of sleep aboard for extended cruise? ----------Do these people realize they are closer to full size cruisers? Than the trawler? I am not a trawler person. -----I have the view that the trawler is a very slow boat on a, NO time schedule. Trawlers are in my opinion, for the retired, or people who can walk from their job for 3 or more weeks at a time. And most important. EXCELLENT boaters in ANY wind or water conditions. These are not the boats for the average docking abilities person. These a marginal powered boats . Not large Searay bow riders with living spaces.
     
  4. Willallison
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    Willallison Senior Member

    Dennis,
    For a start - forget the 32 CheoyLee if you want to go fast. We have a 36 with a pair of 120 lehmans. Way overpowered, it'll do 11 knots (boat was originallly designed for a single 120) but a fabulous boat. In my opinion way ahead of any of the others in terms of build quality. But anyway, I digress - the CheoyLee is a full displacement hull - I doubt you'd fit the horsepower in to do 18 knots! Even if you could - and this goes for all the boats you mention - the fuel burn would be horrendous and the cost of conversion would outweigh the cost of buying a semidisplacement boat in the first place. In general, they have keels that are too big to operate at higher speeds and have considerable rocker in the bottom - meaning that the more power you apply, the only thing that goes higher is the bow!
    In short - if you want to do 15 - 18 knots, buy a boat that's designed to do it.
    As for the DP equipped GB, I'd suggest that the boat would be an absolute dog. Sterndrives generally can't swing a big enough prop to get a decent bite for a boat of that sort of displacement

    Cyclops - not entirely sure what you're getting at when you say
    . Surely you'd agree that the bigger and more comprehensive that a boat becomes, the more sense a trawler makes? Not sure either why you suggest that they're any more difficult to dock ( I would suggest the opposite) or that they are 'marginally' powered...:confused:
     
  5. cyclops
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    cyclops Senior Member

    67-LS1 has already found one condition he does not want. Slow speed in any condition or a true ground speed of 1 or 2 mph in a head on current or wind combined.--------------- I really think he would enjoy some test rides in trawlers and a true cabin cruiser that is even above his price range. Ride EVERYTHING that has the space and power to do what you want in a boat. THEN start downsizeing for whatever reasons. You MUST KNOW what is perfect , to accept a good satisfactory boat in it's place. I can't make it any more truthfull than that.
     
  6. 67-LS1
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    67-LS1 Junior Member

    I guess 12-14 might be beyond reality, but are there things you could do to improve cruise speed at all? 10 knots?

    Are there any other "trawler style" boats that have better hull shapes, ie possibly longer and narrower, that can cruise at a more lively pace? I don't really want a planning hull because I would always be in that akward transistion speed anyway.

    I would like to find something with classic or gracefull lines, like the current trend in picnic boats, without having to break the bank.
     
  7. cyclops
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    cyclops Senior Member

    I can not see what is wrong with having a boat that plows at 12 to 15. Planes at 15 to 25. My boat can do 56 GPS. But i cruise at 20 to 25. and plow at 10, with guests in a 19' bow rider. If a squall appears. Canvas up and home, James.
     
  8. mattotoole
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    mattotoole Senior Member

    You can't fool Mother Nature, or Father Physics. To push a big, fat thing through the water quickly takes a lot of power and fuel.

    Sounds like he was *on* something!

    Just buy a faster boat. What you describe is exactly what most people want, and therefore what Bayliners, etc., are designed for. You're not going to cruise at 12-14kt and burn only a gallon an hour though.

    Before you buy anything, I suggest chartering a few times, so you can see what your needs really are.

    --
     
  9. Robjl
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    Robjl Senior Member

    Dennis,
    The displacement cruiser will travel economically at no more than about (1.3 x squ root of waterline length (in feet) knots.
    You work it out ... if the water line length of your choice is 36'... hull speed will be 7.8 knots.
    Displacement cruisers just won't exceed that speed without massive and I mean massive fuel consumption. But remember if you throttle back to about 6 knots you will use less than half the fuel (compared to the 8 knot load)... that's where the displacement cruiser comes into it's own.
    You won't find a solution to increase the speed as you seem to want... the issue is not the prop efficiency or type or how smooth the hull is or a bigger motor...it will always be waterline length = hull speed.
    I'd be looking for a different kind of hull if you want speed.
    Personally I'd prefer the solid plodder.
    Regards
     
  10. Robjl
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    Robjl Senior Member

    Didn't mention displacement...

    Henry A Scheel designed a beauty that you might like. She gets a description in his book "15 Modern Yacht Designs" Design 218.
    This is a 41' powerboat.
    LOA 41'
    LWL 38'
    Beam 11.5'
    Draft 4'
    Disp 13,000lb
    Apparently one was built.. the owner.."He had nothing but praise for her behavior. She is economical - say 2.5 gals per hour at 12 knots...."
    This boat had an unusual hull shape and was fairly light.
    The motor couldn't have been much more than 120HP.
    She always appealed to me..
    The book is available.
    Regards
     
  11. tom28571
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    tom28571 Senior Member

    This is pretty much the gospel for displacement boats.

    In any displacement boat, the speed range above hull speed just ain't worth the effort and cost. Many "trawler designers" try to go there by flattening out the aft sections but often wind up with a boat that is not very good or economical at any speed.

    True semi-displacement, or semi-planing, boats do operate at speeds into the teens but even they require much power and fuel to do it. The Scheel boat that Robjl discussed sounds extremely efficient, way better than any I know of. If you can find one, you aught to buy it. 2 1/2 gal/hr at 12kts for a 41' 13,000 lb boat driven by only 120 hp is very-very good. Among other things, the secret is light weight. To get that, the light weight mantra must be burned into the forehead of the designer, builder and owner. By the time most buyers are finished with their list of wants and requirements in a their home away from home, the light weight ideal is long gone.

    If it were either easy, or even practically attainable, to get the speed you want out of a typical trawler, there would be lots of them on the market.
     
  12. cyclops
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    cyclops Senior Member

    Buy a cruiser if you are not retired of selfemployed. Trawlers ARE SLOW. When racing. Rent a trawler for a weekend. Should help you find your speed range better.
     
  13. 67-LS1
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    67-LS1 Junior Member

    Is Froude affected by the boats beam? ie would a 40' x 10' displacement powerboat be anymore efficiant then a 40' x 15' boat of the same design?
     
  14. Deering
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    Deering Senior Member

    No and yes...

    Froude applies to the waterline length, so for your two examples your top hull speed will remain about the same. What does change is the power required to reach that hull speed (more for the fat boat).

    Once you get into really skinny hulls, like battle cruisers and catamarans then you're in a different portion of the Froude curve, and wetted surface starts to play a larger role than wave making.
     

  15. Tad
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    Tad Boat Designer

    Tom, good post.

    I would reiterate that speed or performance in powerboats is dependent on waterline length and on weight or displacement equally!

    For example make the GB 32 twice as long but leave D/L ratio the same. Lwl = 30.75', disp. = 17,000, D/L = 261. So a GB 61.5 will disp. about 136,000 pounds. The 136k GB 61.5 will do about 8.8 knots with 122 HP, whereas the GB 32 will do 8.5 using approximately 41 HP. Speed capability has increased slightly.

    But double the length of the GB 32 and leave the weight the same! Now things get interesting. A GB 61.5 at 17,000 pounds will do 8.8 knots using 11 HP, and 12.5 with 44 HP, and 15 knots with 80 HP!

    The Scheel design mentioned above was built by Sonny Hodgdon as Paquet for the Schaffer family. She is light for her length, but I doubt 13k, more like 15,000+. The hull has a hard chine above DWL and is very fine below. Waterlines are narrow and she has a deck line reminiscent of a sailboat. She will be quick under foot, but picks up width aft very quickly with trim. In many ways the form is similar to early Hunt deep-vees, except that this one narrows significantly aft.

    Some quick calc's show 61 HP for 12 knots in a 13,000 pound 38' waterline. This should be about 2.88 usg/hr. The cool thing about this light a hull is that 10 knots would be about 36 HP and 1.75 usg/hr. That is good going.

    Modern "Trawler" buyers are sold something like a Nordic Tug at 32' with 350 HP and capable of 17+ knots. They soon find that 350 HP is expensive to feed, noisy, jerky, and generally not relaxing. So they cruise around at 9 knots using about a quarter of the available HP and pushing a hull that is decidedly inefficient at this speed.

    Take the Nordic's 23,000 pounds and stretch it out to about 60' DWL. The boat could run at 10 knots on maybe 10 HP? Designers have been pushing this idea forever (see LF Herreshoff's Piquant of 1950). Buyer's and Builder's need complete re-education and endless examples. I think it's going to take several more generations to erase the use-it-up hummer mentality.

    All the best, Tad
     
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