4hp outboard

Discussion in 'Multihulls' started by guzzis3, Apr 25, 2021.

  1. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    Some of the boats I've owned over the years had no motor. Those that did had little ones and they hardly ever got used. I can barely remember how any of them performed. I've always sailed out of every situation, but in our litigious society with more and more boats around it's just safer to drop sail and motor when in close proximity.

    Here in Queensland you don't have to register any boat with 4hp or less. You don't need a licence until you go over 6hp. Because of my stroke I will lose my drivers licence and although there is no stated field of vision requirement for the boat licence they may take that from me also.

    I am weighing up whether to build the Woods 25' catamaran which has a 4hp suggested outboard, and the kendrick avalon 9 trimaran. I don't care about running the thing flat out. It's a tool, and a consumable one at that.

    My question is what would be the minimum outboard you would feel is right for particularly the trimaran. In theory a 4hp would push it to hull speed but theory doesn't always pan out.

    It's easy to say: bigger is safer, or something to that effect, but given my situation I need to make some decisions.

    Thoughts appreciated.
     
  2. Kayakmarathon
    Joined: Sep 2014
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    Kayakmarathon Junior Member

    Move to a country where no boating license is needed for your boat.
     
  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Which one of the Woods 25' cats are you thinking of building?
    Sailing Catamarans - First Choose a Design http://sailingcatamarans.com/index.php/designs-2/2-catamarans-under-25ft

    Here is a link to the Avalon 9 metre trimaran for reference -
    Plans for the Avalon 9 metre folding trimaran http://www.teamscarab.com.au/avalon9/design.html

    Re your boating license, do you have to have one if the boat has 6 hp, or more than 6 hp?
    If it is more than 6 hp, then I would have thought that 6 hp should be perfectly fine for powering any of the boats in the above links.
    If you have a good choice of 6 hp engines available in Australia, maybe try to find one which has the largest reduction ratio on the gearbox, allowing it to swing a bigger propeller, and get more torque.
     
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I used to have an O'Day 27 with a 3.5 HP outboard. It was OK unless the wind was on the nose over 29 Kt. However, the boat you describe will have less drag, so a 4HP should be fine.
     
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  5. JimMath
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    JimMath Junior Member

  6. jamez
    Joined: Feb 2007
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    jamez Senior Member

    FWIW My own experience and these are all 2 strokes.
    Wharram Hinemoa: started with a 3.3 Tohatsu. Was certainly adequate top speed about 5.5 knots in a calm and (just) motoring on and off mooring in 20 knots. The 5hp LS I fitted later was better in a number of ways had a remote tank, and the long shaft sucked a lot less air. Top speed went up to about 7 knots in flat water but the big difference was being able to motor into 20 knots or so and chop (this is a common afternoon occurance in Auckland) at a steady 5 knots instead of around 3 knots with the little engine.

    When I launched the 25' tri I had no money to buy another engine so carried the 5hp over to it. The tri has a lot more windage than the Wharram. Same thing, it was adequate. For getting on and off the mooring it was fine. But one day in a flat calm I had to motor 25 miles and to do 5 knots the motor made so much noise (like being shut in a toilet with a lawnmower) at about 80% throttle it drove me nuts. Replaced it with a 9.8 Tohatsu XL - big improvement - I now have 5.5 knots cruise at just under half throttle on a calm day or can punch into 20+ knots at 6 knots if necessary. Top speed in a calm is around 8 knots, but it is noisy/thirsty doing that. I did consider the Tohatsu 6hp 4 stroke Sail, and they are comparable weight wise but got a good deal on the 9.8. It would be interesting to compare the two.

    While one could certainly motor a Scarab 30 with a 6hp I would consider a 9.9 4 stroke to be a minimum for a boat that size, particularly if you want to get in and out of a boat ramp on a crappy day. I like RWs Eagle design (which is the one I assume you are considering) and I would estimate that one of these could be built in half the time/materials/cost of the Scarab. While a 4hp would be adequate I think a 6hp 4s would be just about perfect for the Eagle. It should give a decent cruise speed at moderate throttle and some useful reserve power when needed.
     
    Last edited: Apr 25, 2021
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  7. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    Thank you for the comments.

    The catamaran is a combination of Mr Woods designs. Basically the sango bridgedeck and either wizzer hulls raised with a cabin added or eagle hulls stretched a bit. I need to do some experiments with infusion to see how hard it will be to make curved hulls then I'll decide. Eagle hulls are a tad wider but of course the rounded hulls are prettier.

    The Avalon 9 is straight forward to build and doesn't require me to design and build a special trailer to assemble it. The accommodations are problematic though. Overall cost to build will be similar. The tri is a few thousand more for hulls and sails but I'll probably lose that in the trailer for the catamaran.

    The big issue with trimarans is getting a decent double bed. The Scarab 32 places the V berth further aft. I was going to use the fiberglass beam stubs anyway so I thought if I stretched it 10% to 9.9 and use the S32 bulkhead spacing and rearrange the accommodations to get a decent double forward, a bigger heads area and reduce the settee length.

    I'd use the S32 centerboard and lose the dagger. The S32 is 12' folded so illegal in Australia except behind a truck with wide load signage. Basically what I'd build is a slimmed down S32 using the Avalon hull and float lines.

    I realise a small motor is going to be noisy and have less in reserve. I am visiting my doctor tomorrow so maybe we will have a chance to discuss the licence situation. I just resent giving the government $370 a year for nothing. Trailer registration is separate and another $217.

    The final choice will be up to the boss. I could cruise on a raft but if she is to come with me it needs to be comfortable.

    Quote:
    Re your boating license, do you have to have one if the boat has 6 hp, or more than 6 hp?
    If it is more than 6 hp, then I would have thought that 6 hp should be perfectly fine for powering any of the boats in the above links.
    If you have a good choice of 6 hp engines available in Australia, maybe try to find one which has the largest reduction ratio on the gearbox, allowing it to swing a bigger propeller, and get more torque.

    /Quote

    You need a licence if it's 3.5kW which is a tad over 6hp, so you don't need a licence for 6hp. I suspect the law in Queensland is driven by the tourism industry. Various people here rent boats to tourists who don't have marine licences so I think they provide the exemption for that.

    Tohatsu offer a "saildrive" 6hp 4 stroke here. Long or extra long shaft, can't remember, lower gearing and I think an alternator standard. I was looking at the Yamaha 4hp long with optional alternator as an auxiliary charger for the batteries. It's 6A as I recall, not great but better than nothing.

    https://www.neptunemarine.com.au/motors-on-sale

    The tohatsu 6hp saildrive is about $2295

    Yamaha - Portable 4 Stroke | Brisbane Yamaha https://www.brisbaneyamaha.com.au/yamaha-portable-4-stroke/

    The Yamaha 4hp long is about $1661 and the 6hp is $1939.

    Quote:
    Extended Eagle to 25 ft with cuddy could be powered by a 4hp OB , see Peets' build on the sailing catamarans forum.
    /Quote.

    Yes the outboard mentioned in the sango plans is 4hp.
     
  8. SolGato
    Joined: May 2019
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    SolGato Senior Member

    A lot of times the only differences between the lower horsepower models is the carb.

    For example the Nissan/Tohatsu 5 is the same as the 6, just a different carb which I’m sure just has a bigger main jet.

    The same might be true for the 4 horse as 4/5/6 all tend to look identical.

    When you get into the 8 and up, you see the HD cast aluminum lower legs which results in a huge weight increase, as well as additional cylinders and or carbs.

    If you think about it it makes sense really. Manufacturers don’t want to have to design and build 3 unique models. They just tune them differently to meet requirements like the ones you are trying to meet, or for fuel economy, etc..

    So I would research if a 4 horse is the same looking at specs, and then just buy a bigger carb for it if you need more power. No one will know the wiser.

    Also if I’m not mistaken, from what I understand the Nissan/Tohatsu motors were basically designed by the same designers of the previous series of Honda motors which is why they are so similar.

    You can’t beat the simplicity and lightweight of the Sailpro 4/5/6 motors. And now they have a much improved longer tiller and come with what used to be an optional charging system.

    I’ve been considering ditching my Honda 8 for a Sailpro 6 just to save the extra 50lbs in weight on my fast Trimaran.
     
  9. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    Yes both the yamaha and tohatsu have the same bore and stroke for the 4hp and 6 hp motors, so maybe you can cheat and tune up a 4 to 6hp. The gear ratios appear to be the same also. As far as I can see the only thing special about the sail version tohatsu is the extra long leg, 25".

    Of course such a modification would be illegal. I think I'd see how a 4 goes and then decide. I guess the question was more: is 6 adequate ?or is that going to get me into trouble ?

    I don't envisage launching near the coast. I live near a river. There is a ramp a couple of kilometers from my home. It's a long run down the river but it means the trailer goes straight back home, washed and stored, then I cruise down the river. So launch and recovery is done away from currents tides and wind. I know I can manhandle the boat there, I've launched from there many times before. Anchorages and marinas are another matter but even the trimaran isn't an enormous boat. Maybe 1200 kg sailaway weight.

    Thoughts appreciated. Thank you.
     
  10. SolGato
    Joined: May 2019
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    SolGato Senior Member

    My advice is to look at similar size production multihulls and note what size auxiliary motors they come with.

    For example, with a Corsair F24 Trimaran at 1800lbs it was recommended to run a 6hp and with an F27 an 8hp.

    Both motors are of course side mounted, and from experience I can tell you that it is best to leave the motor fixed and use the rudder to steer.

    I have studied some of Woods designs and he would certainly know what’s best for his boats. I know that with his construction methods and lightweight designs, his power Cats in particular are supposed to be pretty fuel efficient and can run smaller motors, even a pair of smaller motors with excellent results.

    With regard to the Trimaran and sizing, the main issue is windage. Most Trimarans are pretty sleek and low to the waterline, but there are 3 hulls and they tend to sit on top of the water rather than in the water, so they have a tendency to get blown around. This is where having a bit more power helps.

    I have a Nissan 5 on my Farrier Tramp which is a heavy boat for its size compared to today’s designs. It moves the boat around just fine except when in swell with the hull rocking and the prop cavitating.

    I think making sure you have a mounting system that allows for the prop to remain well under the surface will help tremendously.

    With regard to the SailPro, not only is the leg longer but it has a high thrust prop, and there is a model available with the charging system.
     
  11. waterbear
    Joined: Mar 2016
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    waterbear Junior Member

    4hp sounds adequate to me. Woods recommends 4hp for Strider and says to expect 6kts cruise.

    I asked Ferdinand Hartzenberg about the 2hp honda on his Janus 22' and he said 4kts cruise "all day," 6kts max, and 2kts into a 20kt headwind.
     
  12. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Was that 2 knots forwards or backwards ? Just joking, just joking !
     
  13. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    Funny!


    One thing I have been looking into is the noise level of these Little Noise Makers. Allegedly the little Honda 2.3hp is loud even by LNM standards.
    One thing to consider if budget allows and the weight is not an issue is a step up in size. A 4hp LNM at 1/3 - 1/2 throttle makes much less noise than the 2.3hp Honda at 2/3 - 3/4 throttle.
     
    Last edited: May 25, 2021
  14. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    Thank you for the replies.

    I don't "cruise" on the engine. It's only for close manouvers and emergencies, so noise level is pretty irrelevant.

    Anyway we shall see. It's impossible to buy them secondhand here as they are in such demand, so to do the experiment I'd need to buy new.
     

  15. waterbear
    Joined: Mar 2016
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    waterbear Junior Member

    Depends on which way the tide is going :D
     
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