Outboard on a Power Cat

Discussion in 'Powerboats' started by fritzdfk, Dec 26, 2014.

  1. fritzdfk
    Joined: Jul 2004
    Posts: 45
    Likes: 2, Points: 8, Legacy Rep: 32
    Location: Alaska

    fritzdfk Junior Member

    I built a small 18' X 8" power cat designed by Bernt Kohler, a ECO Cruiser 5.5.

    http://www.ikarus342000.com/ECOmotorboat.htm My boat is the red one.

    It has a semi-displacement hull and cruises at 10 knots with a single 20 hp outboard. This has been discussed in a few other threads but I am having problems with ventilation. I initially launched the boat with a self designed fairing in front of the engine which turned out problematic. It was much too wide. Another issue was the motor I bought which had power tilt. The power tilt made the mounting bracket too long causing issues with designing the fairing.

    Anyway I have been trying to sell the outboard, a Suzuki 20hp, a nice engine. I am trying to decide whether or not to go with twin outboards, 2 X 10hp, or another single 20 without power trim and with a 25" shaft. Aftermarket 5" extensions are available for Yamaha and Mercury (mercury includes Tohatsu). Twin outboard would certainly work and eliminate the ventilation issue but would be heavier and add complication and fuel consumption. With a different single engine I would still have the ventilation issue. I have found more information about fairing design for a single engine on a cat and I think it can be done but I suspect that even the best design is not 100%. When it is rough and you really need the engine that is when the ventilation is likely to happen.

    So I am looking for more input concerning a fairing. Or should I just go with twins and pay the weight penalty and complication (and noise?) . The single 20 weighs 120 lbs, twin 10's 180 lbs. I might also mention that I am in Southeast Alaska and and you don't have to go very far to be quite isolated and twins would be a safety factor. With the single engine I would probably want a 4hp kicker so that lessens the weight difference.
  2. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 171, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    The fairing on my Skoota 20 powercat worked well with a 25LS Yamaha at speeds up to 15 knots. We would cruise at 10 in the Strait of Georgia. I had a tilting outboard bracket so I could lower it for more bite if necessary, better than an extra long shaft I thought.

    I see Kohler says you should do 15 knots with a 20hp at 3/4 throttle, not 10 knots. That implies either your boat is massively overweight or you have the wrong propellor


    Richard Woods of Woods Designs

  3. fritzdfk
    Joined: Jul 2004
    Posts: 45
    Likes: 2, Points: 8, Legacy Rep: 32
    Location: Alaska

    fritzdfk Junior Member

    I should have said that I want to cruise at 10 knots. My WOT top speed was close to 15 but I don't want to cruise that fast. I did find a drawing online, Richard, of your Skoota 20 fairing and that would be the basis of a new fairing I would make.

  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,035, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I'd say the type of prop would play a significant part. Stainless with 4 or 5 blades would probably go better, but finding one to fit might be a problem.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.