Conversion from trailer-sailer to low power motor boat

Discussion in 'Boat Design' started by Dr. Peter, Apr 27, 2010.

  1. sltak
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    sltak Junior Member

    Mark 1 Kestrel conversion

    Thanks again for your replies.

    I like the slot top on the Hartley, and appreciate the comments regarding engines. Under water appendages aft is something I have considered too.

    Since the last posting, I have bought a boat, done a quick conversion and just completed a 3-day trial on the upper reaches of the Waikato and Waipa rivers. I can now add a little to this forum.

    I eventually liost patience and bought the first trailer yacht that was reasonably priced. A Mark 3 Kestrel (18' trailer sail boat.) Immediately realised that was a mistake - these boats have a stub keel and require a good launching ramp to float on and off trailer. Sold it same day, and next day bought an old mark 1 Kestrel - somebody had already chopped nto the transom, the sails were past being useable - so a good candidate for conversion. This early version of the Kestrel has no keel, floats in a few inches, and appears to have no ballast either.
    I took out most of the bulkhead at the rear of the cabin, widened the hatch and fitted a couple of moulded ply seats, facing forwards. The seats were picked up for $10. (Nothing "marine" on this baby.) Accomodation for two, and a galley, remain as standard under the foredeck.

    My nice near-new Mercury goes back on the market after discovering it costs nearly $1,000 to fit these things with remotes and teleflex steering. You need special Mercury conversion parts. I have no time (or money) for that sort of nonsense.
    Picked up an old Yamaha 9.9hp 4-stroke that already had remotes, made up a wooden quadrant and fitted her with rope and pulley steering. (They use the same system for the flight controls on light aircraft don't they?)
    Loaded the bow with 50 litres of water, in containers, for ballast - and off up the Waikato. From Ngaruawahia to almost Lake Karepiro took about 7 hours. (I turned around at that point - lesson learnt: the issue is not being able to steam against the fairly rapid current - can do that OK - my fear was running back down the river, going the wrong side of an obstacle, and not being able to stop. As you get further up river fropm Cambridge, the river becomes more and more rapid. The banks looked pretty hard and rocky, and suddenly the old fibreglass hull felt a bit fragile.

    Returned to Ngaruawahia next day, and did a trip up the Waipa tributary. Quite a different river - sluggish at first, very easy - but both sides and sometimes the middle littered with tree trunks and snags, so proceded slowly. Up past Whatawhata it starts to become a bit rapid. The 65' armed and armour-plated paddle-steamer Avon worked right up to here during the Waikato war in 1863/64, well-documented by mishipman Cecil Foljambe. The river appears not to have changed much.

    Conclusion: old trailerable yachts can indeed make excellent little displacement launches. 10hp is enough to draw a sizeable stern wave and I spent most of my time at less than half throttle, with almost full hull speed and no stress. Like the previous posts, I can say 10hp is enough - 15hp might be nice but only because it might be even quieter. I was very happy with my 9.9hp (which has alternator and electric start) and it was powerful enough to push against a fairly rapid current at "the narrows", up-river from Cambridge. Galloping back down with the engine idling just fast enough for steerage was more of a worry. Incidently, I steamed a total of about 15 hours and used a little less than 15 litres of petrol.
    One further point - these motors can be fitted with a fine pitch propellor. It seems you have to retain the same diameter and gear ratio, but at least with fine pitch you can get a bit more useful thrust from your low powered engine, at the relatively low speed of a displacement hull. My motor came with a standard propellor (which I have not tried) and also a fine pitch, which had already been fitted, and was the one I used.

    Being a person who loves sailing, I am not advocating hacking up sailboats, in general. I will retain the sailing ability of the Kestrel, but probably give her a simple and more sensible low rig, as befits a motor-sailer. Twin rudders are all the latest fashion on race boats these days - so I think the motor can stay in the middle, where it should be - and I might put twin rudders on, for sailing, if I get around to making a sailing rig. A shortish mast in a proper tabernacle (not these little stainless steel gismos the trailer-sailers have) will double nicely as a ridgepole - exactly as on the Hartley in the previous post - an excellent idea.

    But there are trailer boats around which will probably never sail again - I actually saw one at the Waitakere rubbish recycling station the other day!
    Such boats can make a lovely trailerable displacement launch and 10hp with a barge prop (fine pitch) is all you need.

    This little old boat will be fine on rivers, estuaries and any sheltered stretches of water accessible by boat ramp.
     
  2. sltak
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    sltak Junior Member

    Mk 1 Kestrel conversion

    Here are a couple of pics.
     

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  3. Dr. Peter
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    Dr. Peter Junior Member

    You have it

    The essence of what we do is about enjoying simple boats. To be honest, my converted Hartley 18 sits in my shed most of the time while I continue to enjoy sailing - but that's OK. Well done, on your conversion.
     
  4. James May
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    James May Junior Member

    sorry for the delay in replying...
    firstly, the 25hp was only brought as it had remotes and electric start... the difference between quarter throttle and full throttle is lots of noise and about 1/2 a knot.
    that said, the boat is awesome to use (if your not in a rush) and the biggest problem i tend to strike is people stopping me to talk about here and take photos wherever we go!!
    SLtak: I wish i could have joined you!!!!!, i cant imagine anything more awesome than going on an expedition up a river or exploring over a few days in my old girl!.
    We also have since brought a little ski boat (kids made me) which tends to be the boat that gets voted to use most often but, it does not compare to floating around on the big boat with the smell of coffee and sausages cooking!!
     
  5. DennisRB
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    DennisRB Senior Member

    Congrats on the conversions!

    Would some designs lend themselves to removal of the keel? Obviously this will be dead weight when towing and motoring and the huge centerboard case takes up a lot of room in a small boat. The mast and sails will be gone so the overturning moment will be gone from that point of view. However as mentioned if the hull sits very high it may become "tippy". It seems to me that some designs may suffer this and some may end up no worse than a powerboat. Would a full stability calculation on the hull be required?

    My dad has an old hard chine plywood 25' trailer sailer. This boat was fitted with a huge oversized 6 foot swing keel of 500kg! It used to plane downwind a breeze due to a fairly narrow stern with a flat run aft so it might be able to exceed hull speed with a decent engine. My dad is getting older and the boat is a bear to rig and subsequently does not get used. Dad used to take it in serious offshore conditions with waves as tall as the mast. But when converted it would be a used in partially smooth waters at most, mainly rivers and estuaries.

    An inboard diesel engine may offset some of the weight of the swing keel. The only prob I see is maintaining beachability with the prop of an inboard which would preclude the idea. Also it seems this boat has a lot if history as a race boat 40 years ago.

    http://www.trailersailerplace.com.au/phpBB3/viewtopic.php?f=37&t=9550
     
  6. PAR
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    PAR Yacht Designer/Builder

    As a rule you can remove only a bit of the ballast, not all of it. In many boats, you lose what you gain. By this I mean what you take off in the rig and sails, gets replaced with a cabin structure, so you can't really remove the ballast. Now, this doesn't mean you can't reposition it, say cutting out a ballasted fin and placing this weight, in the bottom of the bilge. The boat still rides on it's "lines" but you don't have a big 'ol fin dangling under it. The same would be true of your dad's centerboarder. You'll probably still need most, if not all of this weight in the boat, but it doesn't have to live vertically in a case. You can remove the case and arrange the weight horizontally, in the bilge.
     
  7. fredrosse
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    fredrosse USACE Steam

    I converted an 14 ft O'Day Javelin to electric power, removed the centerwell and steel plate centerboard, fiberglassed in a propeller shaft for a 14 inch prop, and added two bilge keels for steering/stability. The boat is very well behaved, and works with 4 adults aboard, moving around at 5 MPH. A larger boat with a few horsepower would behave similarly, with proper bilge keels.

    This build is outlined on the Yahoo "Electric Boats" website, with all details under the files "Building an Electric Javelin", $1000 plus 100 hours work.
     

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  8. sltak
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    sltak Junior Member

    Dennis I think you could remove the heavy centreboard in a hard chine trailer boat. The stability of a boat comes from its hull form in combination with the position of the centre of gravity. A ballasted keel boat is not simply a pendulum. With centreboard boats, launches etc hull form is what gives it the stability - a keel with ballast low down would improve the ability of the boat to right itself from a knock-down, and is usually designed to provide the ultimate ability to recover from a capsize. Maybe your trailer sailer would not recover from a capsize. But if you are using it as a cabin boat with no mast, you are not going to capsize in the normal sheltered waters you would be taking your boat. The removal of the weight will affect the boat - in your case probably for the better, and it may affect the trim. The removal of the mast will give the boat better initial stability and will affect the boat's motion, possibly making it quicker in its rolling motion - you will notice that when beam on to the wake of a speedboat, for example. I would have no hesitation in removing the centreboard and mast on your sort of boat, if that is what you want, and you can always trim it up (as I do on my kestrel) with a couple of well-placed 20 litre water containers (I have to carry mine at the bow.) With the centreboard out and the rigging out, you will have higher intitial stability than before. Pushed over to the point of a capsize, you may or may not be in a better position than before. Most trailer sailers will not recover from a capsize anyway. Howe many launches have you seen capsized? You would have the displacement to spare to put in a small diesel if you want - but any weight you put in your boat will be for trim rather than stability (as long as it is inside the hull.) An easier conversion for a trailer sailer would be a 10 hp outboard mounted on the transom. It is possible that with a larger motor you will be able to defy the theorists and exceed the hull speed (See my next post) but if you really want to go running around at high speed then a runabout would be better.
     
  9. DennisRB
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    DennisRB Senior Member

    sltak, your post pretty much summed up my opinions exactly. However I am concerned that PAR does not agree. I don't really need very high speed, but my observation that this boat can plane undersail with a 500kg centerboard in it, may make it a good candidate for a larger than usual motor.

    The boat is very stable with the centerboard up, even under sail. We always sailed with it up downwind.

    A diesel would be good, but in reality, it will be very difficult to get the shoal draft I want. So a 4 stroke with forward controls might be much better
     
  10. sltak
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    sltak Junior Member

    Over-powering a trailer yacht hull

    Dennis, you've got it. A centreboard is not usually there to make the boat more stable. It is usually there to provide lateral resistance when sailing to windward.
    A ballasted keel provides little self-righting moment until the boat is heeled quite a lot - that is when its "stability" factor kicks in.
    At small angles of heel, hull form is what pushes the boat upright.
    I must say however, that a 6' x 500lb centreboard would give a great deal of self-righting ability in the case of a knockdown.
    Without a rig, however, you should not expect to be knocked down.

    Over powering your trailer yacht hull.
    I converted a Mark-1 Kestrel (19' trailer sailer) into a launch and found that a 9.9hp Yamaha 4 stroke, with remote controls and electric start was a delight, and had all the thrust needed. This motor had more than the usual reduction, making it able to turn a larger than normal propeller - a perfect setup for pushing a launch at displacement speed. The propeller a has less pitch - to be specific I think this 10hp motor is turning a 11.75" diameter x 7" pitch propeller. That is about the ideal.

    In theory the power of a larger motor is wasted on a displacement hull, and for a reasonably heavy hull a ridiculous amount of power would be necessary to make it plane. The previous post describing the Noelex describes this. (You can of course use a larger motor and just make it do less work, thus enjoying the advantage of quieter running, electric start etc.) At higher throttle setting, you would expect anything more than the thrust from a 10hp motor to just cause noise, squatting and excessive stern wave.

    But wait, there's more.

    My so-suitable 9.9hp motor developed a fault so I bought a 15hp Honda to replace it. I was not looking for more speed, only quieter running with even less effort. When the motor arrived I was dismayed to find that it had a standard reduction and a runabout type propeller (9.25 x 9.5). In theory quite unsuitable. And these motors don't come with the reduction or the room to swing a bigger diameter propeller. I was going to have to sell the new motor and look around for another Yamaha "barge model."
    This morning I thought I would just give it a run anyway, and see how bad it really was. To my surprise it was noisier (not quieter) than the Yamaha - but still not bad, both being 4-strokes. It ran quite nicely at low throttle and pushed the boat to hull speed quite easily despite the little propeller. (A cavitation plate is fitted, I don't know if that helped.) But the real surprise came when I "opened her up". She quickly ran up over hull speed and began planing, somewhat heavily, in the manner of a fairly big yacht planing. I would think she was doing 10 or 12 knots - certainly well over hull speed - and without the expected fuss and bother. She pulled a bit of a stern wave, but not too bad - less than a runabout would - certainly not squatting. We had two adults aboard and about 50kg of water in the bow for ballast.
    The only fault I could find was that with the small unsuitable propeller, you can't pull up quickly by engaging reverse, so you lose a little bit there. But going ahead, the extra horsepower was simply converted into extra speed - in contradiction to the conventional wisdom. Also, for this motor, I can get a SLIGHTLY bigger diameter, lower pitch, 4-blade large-blade-area propeller from Solas that I think will give better thrust, especially in reverse, with maybe a little less speed - (though I am not after speed anyway. A runabout is better for that.)

    In conclusion: If your boat can plane under sail, and if you are not carrying too much weight, and if she is trimmed right, it is quite possible you can make it plane by applying a bit more power. I have proved it with the Kestrel using the standard model 15hp Honda.
    On the other hand, for maximum efficiency this boat is better running at hull speed and I think the 9.9hp Yamaha with bigger reduction and bigger diameter "high thrust" propeller is all you need and probably best.

    I think it is most likely that with the rig and heavy swing keel out of your boat, you will get better performance than I can get with the kestrel, (being longer on the water line) and my comments about the above two motors will probably relate equally to your boat (depending on weight I suppose, and assuming that yours is a normal type of trailer yacht hull shape.)
     
  11. sltak
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    sltak Junior Member

    Ballasting.
    Dennis, I meant to add this.
    If you take a 500 lb centreboard out, your boat will certainly float higher. This may or may not be to your liking. If it feels a bit tender, or if the trim is all wrong, you will have to put some ballast back in. But instead of a heavy keel, you can use 20 litre jerry cans of water, placed as low in the hull as you can put them, and forward or aft as necessary to compensate for the weight of the motor at the back. If you anticipate a lot of rolling around in exposed water, then it might be wise to make sure they are well lashed. For rivers and estuaries I don't worry about that.
    Result: the roll/pitch characteristics of your boat will be a little quicker than with a mast and heavy keel, but your stability should be just as good as before, at least with the swing keel in its case - (a kg of lead weighting about the same as a kg of water!) Plus, with a little pump and some plastic hose you have all the water you want for drinking and washing. You fill up at the ramp - and empty out before going home, so easier to tow.
    That's what I am doing anyway. I carry about 40 litres, but it is no problem to sling a bit more in if needed.
     
  12. DennisRB
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    DennisRB Senior Member

    Your experiment with overpowering your TS hull proved that they can plane if the shape is right for it. Knowing this I am sure mine will plane as it done so under sail, and if you take the 500kg centerboard out it should make it very easy to exceed hull speed with say a 30hp. I need to get some pics. The run is very flat aft. It would usually be cruised at below hull speed but being able to sprint when needed would be a huge advantage.

    The only issue as noted will be how it sits with no keel. The boat was once put in the water with no keel years ago. My dad did not recall exactly how it sat, but does not recall any issues. Note the keel on this boat certainly did add a lot of stability. Note it is 500kg!! Not lbs. It was able to be bolted in the down position. I am sure it would have recovered from a capsize with keel so long and heavy on a small boat. This keel was made heavier than the original design. So the boat actually always sat low and dragged the transom.

    A couple of large house batteries would be installed down low. It already has a 100L water bladder. The boat also has an anchor roller with chain and electric anchor winch fitted in the bow. This all adds about 200kg.

    I am sure the boat will sit higher. But I think an experiment would be in order to test how she sits with the keel removed before any other mods are performed. It may be OK, or maybe not.
     
  13. sltak
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    sltak Junior Member

    Dennis, what is the design does it have a name? Do you know who designed it?

    Yes you are right, with that sort of keel (other things being equal) she would probably come up from a capsize and drag the mast and sails up again too. But that sort of ultimate stability is not usually called for in a launch, or for that matter a small sail boat like a Hartley trailer-sailer, for example. An even better example is a catamaran - very high initial stability, with no keel or any ballast. A motor catamaran would be very hard to capsize. Of course, once upside down, it becomes even more stable!

    It will sit all right. 500kg is a lot to take out, but I am sure if you put some weight back in, down low, like your batteries, water etc. you will get it sitting nicely. By the way, in answer to the majority who will tell you that the keel is necessary for stability - until about 100 or so years ago, the age of sail, ballasted keels were unheard of. All ballast was carried inside, and ships did not have appendage keels. Hull shape which has low inherent stability and is designed for a deep ballasted keel is slightly different, but a trailerable boat with a lifting keel will have enough initial stability, especially with no mast. If not, and if all else fails, a few bags of sand under the floorboards is not unheard of, as in the old mullet boats for example. All this assumes you are motoring in sheltered waters. If ballast shifts, that can change things rapidly for the worse.

    A bit of weight at the bow or stern will soon get it trimmed right, though I would think your keel would have been roughly over the centre of gravity anyway - if it is out of trim that would more likely be from putting a big motor on the back, not too hard to trim out. A bit of trial and error and you will soon get it right. If it is a bit tender (tip easily if you stand on the gunnel) you will soon know, and a bit more ballast will help. But with the mast gone I would think it would be stiffer, if anything.

    The danger is if it is cranky (very slow to recover when it rolls one way) - that can be a danger signal. When you turn sharply and it rolls out, and does not seem to recover quickly - that's a sign things are not good. I saw a small launch heading off to Waiheke with a deck load of bags of cement once, that was cranky like that. In my opinion that was dangerous. I really don't think you will have anything like that to worry about at all.

    I would just take it one step at a time with the ballast, until it feels and looks right.

    Same with the motor - is there some way you could try something a little less than 30hp to start with? You are getting into the runabout category with 30hp. Make sure the transom is strong enough to withstand the thrust (and the stresses while towing) of such a big motor.
     
  14. sltak
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    sltak Junior Member

    I have just gone back and re-read what PAR wrote. It makes perfect sense, and seems quite consistent with what you are proposing - ie you take the ballasted swing keel out and put at least some of the weight back in, in in the bilges, in some other form. If it was over-ballasted and dragging its stern before, then presumably you will want to put only some of the weight back. And if it is in a form where you can dispose of some of it at the ramp - then you will have gained a lot in terms of ease of towing.
     

  15. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Hi All,

    This is my first time on a forum so bare with me.I have modified two trailer sailors, one to a power launch using a Frank Pelin 17 ft hull and the second a 16 ft Hartley GTS (Hartley 16 fibre glass version) which I raised the cabin roof and converted to a motorsailor. Both boats I kept the swing keels but both boats only reached hullspeed which at 5 to 6 knots is painfully slow.I have purchased another 16ft Quicksilver same as the hartley 16 fibreglass version.I intend to modify the hull this time and remove the keel and try and get the hull lines as close as i can to John Atkins Ninigret.
    Sorry not trying to break up the thread but I have been there as above I have changed motors,props and mounting positions all end up with the same hull speed.I suggest that the easiest thing to do would put a wedge aft to change the buttock angle.I have seen this done but don't know how efficient it is so I am going to modify mine from amidships to the stern.Then you could increase your motor to maybe a 15hp. Sorry I don't know how I got here i was supposed to be on the thread with the Hartley 18.
     
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