Thoughts or Counsel Solicited-Fisher Potter Ketch Motorsailer Converted to Outboard Trawler

Discussion in 'Boat Design' started by JAXMKE, May 12, 2018.

    Joined: May 2018
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    Location: Jacksonville, Florida

    JAXMKE New Member

    I just joined the group. Hello.

    I came across this forum while searching the internet for perspectives on the questions and thoughts below. And since I have worn out my friends and family talking about boats, I thought I would reach out here. And, as I reread what I wrote, I am starting to think I might have placed it in the wrong category. Apologize in advance if I did.

    I have a Fisher Potter Ketch Motorsailer that I have decided to convert to strictly a power boat. Others have done the same, principally in the UK.

    Part of my conversion/restoration work involves removing related thru hulls, propellor shafting, exterior teak trim and other items that are maintenance dependent and that have generated many, many small leaks over the years. Essentially it is a conversion of a small sailing yacht to a simple workboat. I'm done with the demolition components and am 2/3's finished with the build back.

    My strategy for propulsion involves using two separate 1990's Mercury Mariner 15 HP two stroke remote control outboards that I already own. They are to be located in separate wells in the stern lazarette/bench. The location of the wells are forward of the transom and transom-hung rudder. The motors have extra long 25" shafts so they go down a bit, but not quite to the depth of where the original propellor was. I have the ventilation and motor raising issues inherent with outboard wells worked out.

    I came up with this two-motor two-outboard well design because I wanted to preserve the look of the original transom and wanted easy-to-remove, easy-to-source, lightweight, cheap and redundant propulsion. So, it is what I am doing and I am doing it purposefully.

    More than anything, I have speculated that this design will make it easier to back out of my slip in my tight marina. I think that because I can reverse one motor with the other motor running a little in forward that this might help with rotating this extra-heavy, wide-beam, long-keel lug-of-a-boat out of the slip and turn before I hit the boats behind me.

    I am soliciting counsel on whether this engine set-up will help with rotating the boat in a material way while backing.

    An additional component to the design would be to add a bow thruster. I don't feel like doing the extra work and installation, yet I am not ruling it out. If there is consensus the twin motors alone will help materially with backward turning, then I will cross it off of the list.

    The second piece I am wondering about is whether two 15 HP outboards are enough power. I think they are, but the question makes me uneasy.

    The boat displaces about five tons. I have read with a displacement hull there needs to be two horsepower per ton to attain cruising speed and three horsepower for hull speed. If this is so, one motor at full throttle should get it to hull speed. And also if this is so, then I have my redundancy issue worked out. Does the group think this sounds right? Or, bigger motors? Perhaps late 1990's/early 2000's two stroke extra long shaft Yamaha 25 HP motors? They are tremendously reliable, a small package and OEM supported, much like Mercury.

    I have other sailboats; some with diesels, some with outboards and I really can't tell the difference between a Nissan 8 HP two stroke outboard pushing one of my folkboats and the Yanmar 1GM1o pushing the other, although I understand there is the question of torque which I don't grasp well.

    The third thing I am wondering about is stability. Without the sails I predict the boat will be a little tippy, maybe 10-15 degrees in bumpy water. I think I am talking about primary stability in this vein. After the 15 degrees, the several thousand pounds of ballast in the keel should counteract any further heel (if that is the right word). It's the quick side-to-side 10 to 15 degree roll I want to dampen.

    To improve what I think is primary stability, I believe I have two affordable options.

    The first is to make stabilizing fins and attach them to the hull of each side of the boat. I don't like this idea because I think the travel-lift operator will eventually catch them in his slings and cause damage. Plus, they would be through-bolted in the hull and I'd eventually have leaking which I am trying to counteract almost permanently with everything else.

    The second stabilizing option I am considering are paravanes. Thirty years ago when I worked in the commercial fishing industry in Prince William Sound, Alaska, I remember many boats used paravanes, whether at anchor or traveling from point A to point B. I have not been up there in a long time so I don't know if they are as prevalent, but I recall launching and retrieving them myself both in good weather and bad and it was never a problem and they worked well (especially) at anchor.

    I could weld up a set of paravanes, masts, booms, etc. with spares and bits I already have. My guess is the extra gear would add another 350 pounds to the boat and all of that above the waterline and toward the stern which is already getting heavy from 175 pounds of new outboard motor placement.

    As to the paravane placement, I have read they should be placed no more than 25-30% forward of the rudder otherwise they foul up the steering. So, with proper placement, the stern will be substantially heavier than the original design considering the addition of outboards, paravane masts, cables, chains and booms back there.

    Adding to the aft-end load, if I designed the paravanes to 80-100 square inches of surface area each, I have read the down-force would be around 5-10 pounds per square inch at five knots or up to 1,800 pounds of down-force. This seems like it would swamp the stern of the boat.

    But, who knows about all of the math, buoyancy calculations and loading the original designer considered would be affected by the aft mast and sail that is no-longer there?

    Some may say paravanes are a pain to deploy. It is work and they do take a moment to deploy, but a lot less time than setting three sails back when this boat was a ketch. So the extra work associated with using paravanes, compared to setting sails is nil in my experience.

    I wonder if the group thinks this paravane idea is worth continuing to consider or if the side fins would be a better choice? Or, perhaps just enjoy the boat as is with only the outboard motor conversion and maybe bow thruster?

    Incidentally, our cruising grounds are from the Saint John's River Inlet at Jacksonville, Florida up the Saint Johns River to Sanford, Florida. That's about 130 miles of winding river and hidden paradise. Sometimes the water is rough from the tide and river current in the downtown Jacksonville area, but most of the time the seas are one foot or less DSCN0859.JPG DSCN0826.JPG DSCN0859.JPG , essentially they are just calm. However, the occasional go-fast power boaters and big motor yachts kick up some huge wakes which are the main reasons driving the need to find a way counteract or dampen roll.

    Thanks for reading and your consideration.

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    Last edited: May 12, 2018
  2. Richard Woods
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    Richard Woods Woods Designs

    In haste. I think bilge keels would probably stabilise the roll better than paravanes, particularly in the St John river (which I know well north of the Orangedale bridge)

    I would be wary of 2 st 15hp outboards. Unlike to have the right gearing/props for slow speed heavy displacement. Better choice would be a 25 bigfoot merc or Yamaha HT (maybe twin 9.9) I also suspect that there won't be enough air into the lazerette for 2 sts to run at idle

    Twins will make it easier to manouver

    Richard Woods
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    JAXMKE New Member

    Thank you Richard for your consideration, ideas and judgment in this matter. My home port is Old Bull Bay just 14 miles north of the point you mentioned.

    By the way, I have enjoyed studying your website this morning. The Duo Dinghy story has my attention at the moment. Versatile, lightweight and fast rowing.

    Thanks again, Jerry
  4. gonzo
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    gonzo Senior Member

    If I remember correctly, they were originally fitted with a 27HP Yanmar, so the total power is OK. However, as Richard pointed out, they may not be geared for low speed. I'm not sure if they made high ratio gears for those motors. If you are in the St. John's river, call my buddy Rob at Boatwrench in Longwood. He should be able to help you.
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    JAXMKE New Member

    Thank you Gonzo. Yes, you are right on 27 HP Yanmar.

    Those two stroke 15 HP Mercury Mariners do not have the numerically high gear ratios of the modern high-torque outboards with strong low ends. Since reading your post; however, I have found that propellors are available for my motors with varying pitch which I think might accomplish somewhat of the same result--leveraging what torque those motors develop. Although it might be something like converting a sow's ear to a silk purse.

    I was looking at the Yamaha high torque 9.9 HP outboards that Richard mentioned and it is pretty clear that no amount of propelloring will get my Mariners to perform like the Yamaha four stroke.

    I will look up Rob. Thank you for the referral. Jerry
  6. JSL
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    JSL Senior Member

    check out your prop blade area
    assuming the original inboard diesel was sufficient (and safe*), your outboards should have the equivalent blade area. The prop and gearing etc. simply may not be available on outboards.
    Safe*??? Years ago I saw a similar 'small blade-big boat' situation. Given time, the outboards would get the boat up to reasonable speed but when it came to stopping and maneuvering it was quite a gong show.
  7. RHP
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    RHP Senior Member

  8. wayne nicol
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    Location: Queen Charlotte islands, B.C.

    wayne nicol Senior Member

    i live up here on the queen charlotte islands/ Haida gwaii. NW BC, and all the fish boats up here- and some of the rec. boats use paravanes.
    i can not comment as to what might work better or not- but all i can say, is that all the pro's still use them up here.
    what are the options of putting the booms more midships, but angled back so that the actual paravanes are in the water at the correct place, that may distribute the weight better.

    good luck, looks like an awesome little boat, and a neat conversion!
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    JAXMKE New Member

    Thank you JSL, RHP and Wayne for your insights, observations and recommendations. And thanks Wayne for your compliment on the boat. I really like it and I often go outside just to look at it. I'm also am pretty excited about its new life and deployment as a river trawler. Regardless, I'm going to hold on the spars, sails, etc., etc. just in case I got it all wrong.

  10. Bobhjax
    Joined: Sep 2022
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    Location: Jacksonville

    Bobhjax New Member

    I’m also in Jax in have been looking for one of these…just found one up north and will be traveling up to see her….do you still own yours? Would it be possible to see it and maybe save a long drive? Thanks Bob
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