will a 20ft wide powercat run on one engine?

Discussion in 'Boat Design' started by Frosty, Apr 3, 2010.

  1. Frosty

    Frosty Previous Member

    What do you think?

    Will a 20 foot wide power cat be able to run on one engine or will it go round in circles.

    One engine is dead making the prop drag and making things even worse.

    Is'nt that why power cats are not made so wide?
  2. messabout
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    messabout Senior Member

    The wide cat would be able to run on a single engine at low speed. Thrust on one side and drag on the other would increase in some proportion to increase in speed. That proportion might well be an exponential one.

    That phenomena may be one of several reasons to put some limits on width. I suspect that the main reason is concern with structural integrity. Imagine a pair of bouyant structures seperated by a large distance. Now imagine driving the bouyant pair through a quartering wave. The connecting structure between the two boats could be subjected to very serious twisting forces. The problem would become disproportionately more serious as distances between hulls increase. Dockage fees and slip widths may very well play into this scenario also.

    Another reason is that a 20 foot wide boat will have a problem trying to go between bridge abutments that are seperated by 18 feet and it could be a bit tedious when moving through a very crowded anchorage area.

    This is another of the infamous boating dilemmas. The wide cat would certainly be advantageous in terms of living space, more deck space, port and starboard staterooms would provide more privacy, etc. But then there are the disadvantages as above.
  3. Frosty

    Frosty Previous Member

    Ive never seen a wide sailing type power cat. Its my opinion that beside all the obvious draw backs such as berthing hauling etc, I think that the boat will not be able to get home with only one engine.

    An acquaintance of mine has just designed --himself-- a power cat of 20 ft beam, its almost finished.

    It may,-- just,-- Im not quite sure but it will be difficult I would think.

    With both rudders way over( more drag) and as you say the prop drag on one side ( which will be 2 foot and a 4 blader)!!!!!! I dont think so.
  4. Typhoon
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    Typhoon Senior Member

    A good friend's family once owned a 32 foot sailing cat, it has a 17hp Volvo diesel in teh starboard hull, driving a conventional shaft in that hull only. It had conventiona, very full displacement hulls for a catamaran (Eureka 32).
    It did have yaw on initial power application, but as soon as you got propwash and about a half knot of forward motion going, she'd straighten up and answer teh helm no problems. At full speed, she'd carry maybe two inches of wheel over from dead ahead, if that.
    Reverse was a little worse but no problem once steerage way was achieved.
    Now, it was actually quite an interesting boat to manouvre, and not at all difficult, once you understood it.
    Being a cat, with one engine in one hull, you had excellent manouvreability using engine thrust, but it had to be thought out.
    You came into a wharf at a fairly steep angle, carrying some decent speed, then when you selected reverse, you judiciously applied power so as to arrest forward motion whilst turning the boat via the powerful lever that was the engine in one hull.
    If you got it wrong, the boat simply laid too far off the wharf, and a quick burst of full throttle forward, would kick the bows of the boat to port, whilst teh propwash also initially kicked the sterns to port. So, once you got used to it, it could be manouvred very, very well. If you got it wrong and enede up putting the bow up against the wharf, you'd just secure the bow with a rope and walk the sterns across with power.
    I also picked up moorings in that boat in anything up to 30-40kts with no issues, again, had to plan the overshoot/failure so you could get out of it, but no problems and I don't recall ever hitting anything heavily!
    I wouldn't hesitate to build a displacement cat with one engine after this experience, although I may consider a small thruster in teh engine carrying hull for extra manouvreability.
    If you want to experience it, do a saddle tow of two boats of similar size and displacement, it's very much the same.

    Regards, Andrew.
    1 person likes this.
  5. Fanie
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    Fanie Fanie

    Why don't you ty it on your boat. You can always pretend the one yanmar is dead. Too many power wires into the boat eh :D

    The single engine option is usually considered the limp home option, ie one engine failed, but you got the answer already. Some sailboats have one engine only and it gets positioned between the two hulls so it won't steer like a crab. It does make maneuvering more difficult though.

    Like Andrew said, once the hulls get a bit of speed they want to slide foreward through the water and not sideways where there's a lot more resistance.

    Are you selling the one yanmar ? :D
  6. CDK
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    CDK retired engineer

    The power cat on one engine will prefer to move in a circle of course, but if the rudder area is sufficient it can also be forced to follow a straight line with considerable drift towards the passive side. So it can limp home in calm weather.

    The drift is caused by the vector resultant on the rudder and is present at any power setting: if the sideways component is 30% of the thrust, doubling the thrust will also double the drift.

    I have made lots of tests to determine the minimum rudder area on my boat. Running on one engine through a narrow channel, from the helm it seems to go straight ahead but still slowly approaches the shore. The wake shows that the boat follows a snake-like path, similar to manoeuvering in strong cross wind.
  7. mark775

    mark775 Guest

    Frosty, there is a small powercat company here, Glacier Bay. They recently went bankrupt so may be difficult to find. Good, displacement cats. The guy is making single engine cruisers and started up again, I am told. They burn so little fuel that you have to pump it off to avoid a spill, with no problems whatsoever - mind you, a displacement cat.
    Hey, it looks like somebody else bought the name and some molds... http://www.glacierbaycats.com/ but it is the old owner doing this. I can find, if you want. I know a guy that knows him.
  8. masalai
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    masalai masalai

    Hey Frosty, are you going to buy a different/new boat? Is it an age issue (yours or the boat?)

    What are your new boat criteria? Come on, give us all something to pontificate on...

    Do the analysis on your own board and put the link here? Questions, Questions Questions where is the drivel? mellowing with maturity? :D
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    FAST FRED Senior Member

    Perhaps the combination of prop walk and slightly angleing the shaft could be used so the helm is straight at whatever cruise speed is chosen as Normal Cruise.

  10. masalai
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    masalai masalai

    I felt likewise but it was suggested that such was not necessary and would cause other problems so not worth while... (thrust distribution, alignment and other issues...)
  11. Steve W
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    Steve W Senior Member

    I owned a Macgregor 36 sailing cat which was 18ft wide and with a single 9.9 yamaha on the stbd transom and it motored just fine with just a little tiller offset,no more than most boats have for weather helm when sailing. As far as the situation you are talking about i would think that the drag of the prop of the dead engine would cause less problems than you would think,after all it is routine for powercats to run on one engine, alternating daily,when passagemaking, for economy and range as well engine longevity.I just recently read an article in Power multihulls magazine about the Moorings 37ft powercat of which something like 18 have been delivered on their own bottoms from Cape Town to the west indies,they talked about the measures taken to get the range needed, running on one engine at a time being one of them.
  12. Frosty

    Frosty Previous Member

    Ok my boat is 5,3 meter beam and yes I can run on one engine. My hull centers are only 3.8 meter, but this boat of my acquaintance is 20 foot beam. Power cats do not have big keels or dagger boards to assist in keeping a line and have smaller rudders.

    He has hard chine hulls and 2x 250Hp, so big props!! and Im just wondering if his design will be a failure,--well in so much as getting home on one engine.

    44 foot LOA at 10tons, 20 foot wide, hard chine!! I think it will go round in circles

  13. masalai
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    masalai masalai

    VERY BIG ONES like the circumference of this globe... Well not really but I do not think he will have much of a problem, except he is quite underpowered when compared to yours? or is his boat 50% fuel capacity in max weight (around 4tonnes without fuel? and 2 tonnes of personal effects?)
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