Tollycraft 26 hp reduction

Discussion in 'Powerboats' started by Zappi, Sep 26, 2011.

  1. Zappi
    Joined: Oct 2008
    Posts: 103
    Likes: 2, Points: 0, Legacy Rep: 31
    Location: Puget Sound

    Zappi Senior Member

    I have a standard issue 1974 Tollycraft 26. She is about 8' beam at the waterline and 24' of waterline weighing in at a whopping 10,500 lbs. An Ed Monk design "semi displacement" with immersed transom and protective keel. There is a Mercruiser 350 MPI 300 coupled to a mercruiser vdrive/transmission.
    Motor is in need of replacement. I'm considering a diesel swap but going with drastically less hp for maximum fuel efficiency. How do I figure out or can someone figure it for me???... How much horsepower do we need for a comfortable minimum? Ideally I would like to cruise at ten knots with a top speed of 12ish? And a ridiculously efficient speed of 7 knots for super long range. We're thinking on making this an inside passage trawler type boat. Thank you and flame away
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,174
    Likes: 965, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    What was your cruise speed with the existing engine ?
     
  3. Tad
    Joined: Mar 2002
    Posts: 2,308
    Likes: 192, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    10 knots will require about 55HP and 12 knots about 90Hp at the propeller with a clean bottom in flat water and no wind. 7 knots will require less than 30Hp which may be a high-idle for the 100+ you would install to get 12 knots, this may not be workable for long trips and most manufacturers will warn against it.
     
    1 person likes this.
  4. Zappi
    Joined: Oct 2008
    Posts: 103
    Likes: 2, Points: 0, Legacy Rep: 31
    Location: Puget Sound

    Zappi Senior Member

    I purchased the boat with a non running engine but it's a common boat and power that it had. I believe it cruised about 15 knots at around 3100 rpms with the 300 hp SBC.

    Tad, thank you for your reply. Being a BC resident I would bet you know the Tolly 26 pretty well. Would it be ideal to find something in the 55 to 75 hp range to keep it loaded at lower speeds and still cruise at ten?
    Another crazy question maybe fit for a new thread. Can full width trim tabs increase waterline? Can they create a slight rocker effect to the bottom if the were say 16" plus in length? I was picturing two simple full beam tabs as large as physically possible with out a need for beefing up the transom at the mounting point. Then non adjustable other than maybe 6 turnbuckles. If they were adjusted up a bit could this benefit the turbulence factor at the stern???
    Thanks All...
     
  5. Tad
    Joined: Mar 2002
    Posts: 2,308
    Likes: 192, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Trim tabs will do little or nothing as long as the transom stays "wet", which it will do at 7-10 knots. They may make a tiny difference at 10 knots as the boat starts to trim stern down.

    Something in the 50-80 HP range would be reasonable, you may have to adjust for a decrease in weight.
     

  6. Zappi
    Joined: Oct 2008
    Posts: 103
    Likes: 2, Points: 0, Legacy Rep: 31
    Location: Puget Sound

    Zappi Senior Member

    Found a Nissan sd33t which I think will be the candidate for the swap. 101 hp and 672 lbs. I'd like to think this will be a fun project. Thinking keel cooled. Any suggestions?
     
Loading...
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.