Friday, July 20, 2018

Mini Max hydroplane

Update on the virus warning in the comments:
I just received a mail from the administrator of the Muskoka seaflea page, and they were attacked heavily about a week ago. They have had it all cleared, so it should be safe now. 
Apparently a warning can "hang" in the system until you update your browser?
Quote: "some times the cache on a visitor's browser will hang onto the "warning" until their browsing cache is refreshed"

Thanks to a much better than normal summer (hottest in Denmark in 150 years or so), There might be a real chance of some time spent at the beach.

Gustav has long complained that we haven't got a boat. When I tried to tell him that we could probably get a rowing boat or a small sailing dingy, he looked at me the way only a frustrated teenager can do. The look that clearly tells you that in his opinion you must be living in the 20th century (which happens to be correct for my part)

Sitting out here without a definite woodworking project going on, I started surfing the web for some inspiration. I have always admired those small hydroplanes from the 60'ies. Those on the cover of Popular Mechanics etc. Nice looking cover articles from a time when the western world was pastel coloured - and smoking and drinking was recommended by your local doctor as a perfectly legal way to wind down after a long day at work.

There are a couple of places that has those old designs available as free pdf files for downloading.
I ended up at a place called Muskoka seaflea, they had just the right plan for me.

The incredible Mini Max Hydroplane.

It is made out of two sheets of 1/4" plywood, a bit of regular wood, a little bit of epoxy to seal the edges and that is about it. According to the article, you can build it in two days, or one day if you have some woodworking experience.

Back in the days this little boat could apparently be built for 20$. That figure doesn't quite get you there today, but I have made a loose budget, and I think that I can probably build it for something like 200$. The most expensive stuff being the epoxy and the paint.

We have an old 4 hp outboard engine, and that will have to make do for a start. If it is a success, I might have to look into finding a more powerful motor later on.

I have spent a bit of time figuring out the radius of the curves shown on page 2 of the instructions.
Those old plans can seem a bit vague compared to what is available nowadays, but since they are free you can't really complain. And if it was possible to build one 50 years ago based on those plans - I can't see why we can't do it today.

According to my calculations, the radius of the "fore center strut" is 85.4"
The lower radius of the "fore cross piece" is 86.2"
The upper radius of the "fore cross piece" is 102.1"
We'll have to see if I am correct once I start building the boat.

The plan is off course to get the boys involved. The individual pieces are not very heavy, and today with the possibility of using screws instead of nails, the buys can really play their part.
If all goes as planned, I will be back home Tuesday night, so Wednesday would be a logical starting point with a trip to the lumberyard for some plywood and possibly some screws.

In the free world, it is probably legitimate to let your children play in a speed boat. But guess which country that has regulations for that as well..
But being the less than enthusiastic citizen that I am, I looked at the official page from Søfartsstyrelsen (the Danish equivalent of US Coast Guard) to see if the rules were possible to work around.
The rules state that you have to be 16 and have a speed boat licence if the power of the boat is more than "the square of the length +3".
If the boat is of a planing type and it is shorter than 4 m (~13'), you need to be 16 and have a licence if the engine has more than 19 hp.

So by making a really short hydroplane I elegantly manage to circumvent the regulations, and can let my kids use the boat to their hearts desire. As long as I stay below 19 hp. But given that the design suggests a maximum of 15 hp that shouldn't be a problem.


  1. Thanks... but a heads up.
    I have two different antivirus programs telling me that the seaflea site is infected with a virus. You may wish to contact the site host and let him know.

    1. Hi Gary
      Thanks for the comment.
      I'll tell the guy running the site about it immediately.
      On another note, I think that you can find the same plans if you search for Svenssons free boat plans.

  2. With the comment about viruses, I didn't look at the plans.

    Curves are probably done with (at least) 3 point materialised by 3 nails and then curving a wooden lath against them. Then no need to know the curvature nor where the center might be. Added benefit: it should ensure that the (ply)wood is able to follow the curve.

    look around 55" on this video:
    or this one:

    1. Hi Sylvain
      I had not idea about any viruses, but you can find the same plans if you search for "svenssons free boat plans"

      The nail/batten trick was how I wanted to proceed in the beginning, but since it should be a curve, I thought that I might as well whip out a bit of the old geometry lessons and and try to calculate the radius.

      The curves are very gentle, so I don't think it'll be a problem for the plywood to wrap around them.

  3. Rowing and sailing may look old school but it comes handy when the engine refuses to work.
    Your kids should know how to row and even to scull ("godiller" in French) with a single paddle.
    Do you plan to stuff the little boat with some plastic foam to make it un-submersible?
    A word of caution: it seems some expandable foams in spray-can are able to exert quite a pressure. Boat design consider pressure from outside and generally not much from inside.

    1. We will make a paddle, so there will be some an "emergency propulsion".
      I'l also have a talk with the boys about just sailing along the coast and keep pretty near to where we will set up base camp on the beach. The boat is for flat water only. It is not at all for rough seas.
      I don't plan on stuffing the boat, but it is built with 3 individual airtight chambers. The outer seams are strengthened with a strip of fiber glass tape and some epoxy.
      The boat will be lifted ashore when it is not in use, so there won't be a risk of it slowly filling up with water.

      I am afraid that if I was going to use expandable foam, it would indeed press the upper hull from the lower. I suppose that I could get my hands on some styroform and put that inside the airtight compartments, but I really don't think it will be needed.

  4. Jonas,

    Your projects always sound like fun as does this one. Even better it should be a good reward for the kids at the end.

    Photos to follow?


    1. Hi Ken

      Thanks for the nice comment.
      I don't have any pictures yet, and I thought that it was to stretch it a bit to use pictures from someone else's page without obtaining permission :-)
      But Wednesday should see the start of the build, and hopefully some pictures too.

      Gustav is thrilled that we will make a hydroplane. If it will be a success, we might have to build one more, so each of the boys can have his own.


  5. There is not much I could find on Instagram of your minimax, but there are lots of videos on YouTube, including a build tutorial from the website you lost. That thing looks fun! Will you rig it with a steering wheel?

    1. Hi Brian

      I never even thought about looking at Instagram for a Mini Max.

      I think that we will try to rig a steering wheel as well, it would make it more fun in my opinion.
      But I am considering to make a small rudder and leave the outboard motor fixed. That would make it a lot easier on the remote control of the gear lever and the throttle without having to deal with the motor itself is turning.

      But we might also just start out doing the bare basics, and then get on the water as fast as we can.


  6. My boss, Jim and his daughter built a mini-max eight years ago. They ran and enjoyed it immensely. Jim built one as a teen ions ago.
    Sorry, I tried to include a photo taken this spring, but it would print in.
    I think that Jim made the motor mount taller to bring the prop up close to the bottom plane. More speed, less drag.
    There is another group who have been gaining traction on the east coast. Look at :
    Cocktail Class racer.
    It is a more involved build, but food for thought.

    1. Hi Paul.

      Thanks for commenting.
      I don't think it is possible to include photos in the comments on a "blogspot" platform, but thanks for trying.

      Thanks for the tip on the motor mount. I am not sure if the old motor that we have at home is with a short or a long leg, so I'll try to sort it out.

      Those cocktail class racers look awesome! But I still think that I'd better start with a Mini Max, just to get the hang of it.

      There is to my knowledge no tradition for this type of craft in Denmark, but I could really see the possibilities of having a great tie in a small boat that doesn't cost a fortune to build.


    2. Hey Jonas,
      One construction detail I remember. When you want to bend the plywood to join the bow together, block the outside edges up and walk up the centerline. The panel bends and the vee joint comes together . Jim used a piece of steel strap , through fastened out at the front edge to hold the joint together while glassing it.

    3. Hi Paul

      Thanks for the tip.
      They suggest a bit of a different approach in the original article, but I saw a slideshow from Muskoka Seaflea, where they admitted that it was probably the vaguest possible instructions of the entire build.
      Sort of: "close up the gap and mount the center piece, use a piece of rope if you have to."
      In real life they said that this part was the trickiest part of the build, but they got by using clamps.

      I think your boss's method sounds very interesting.
      My idea was to use some clamps and then force the clamps together with a cargo securing strap. But like they say:
      There are many ways to skin a cat :-)

      Fiber glass is something I have very little experience in using. I can actually only remember having used a product called "Thistlebond", that was designed to make emergency repairs on broken pipes such as cooling water pipes etc. on board a ship.
      I found that the epoxy set ultra fast, so I have purchased some slow setting epoxy from the West system. That should give me around 20 minutes of open time. I still might have to mix a couple of batches though.


  7. Jonas,

    I just finished looking at several build videos of small boats and a Mini Max build (your link) both the build and use looks like a hoot.


    1. Hi Ken

      I think it is going to be a blast!
      Perhaps you should make one to bring along in the motorhome? Potential fun for all the grand peanuts who might tour the road with you :-)

      What I really like about it is that there is so little material used. And it seems genuinely suited for someone wanting to get on the water as fast as possible rather than fuzz over small details (those boats have their place too, but just not with kids involved in the building process).

      I am also amazed that the weight of the boats is supposed to be around 70 Lbs. So handling the boat out of the water shouldn't be too hard.

      Last night I contemplated over how to make the connection for the gear lever and the throttle without having to fork out a lot of money for a commercial set.
      I ended up deciding to use a cheap style lawn mower (walk behind model) throttle. Those have a flexible cable and a solid piano wire core, so they can push and pull.
      I think I have at least one old mower engine at home with one of those on, but I have a friend who repairs old mowers, so I can probably get a couple from him too.


  8. Some thought about using a trammel/compas or nails and a lath.
    The compas will give you a constant curvature arc.
    With the nails and lath:
    If you use 3 nails numbered 1, 2 and 3, as there is nothing to curve the lath before nail one and after nail 3, the curvature is null at those two points and increase while going in direction of the middle nail #2. I guess the curvature is maximum at nail #2 (at least if nail #2 is equidistant from nails #1 and #3)
    So one method is not a substitute for the other. verify on the plans if it is done for the nails/lath method.

    I have seen a picture on the site "svenssons free boat plans" and I noticed the engine placed rather high.
    I guess if the propeller is too low it will tend to lift the bow too much and make the boat difficult to control.
    There must be an optimum between (boat-)planing/surfing (déjeaugeage in French) and control. It might also depend on where you sit and how much you weight (center of gravity) and also the weight of the engine.
    A rudder would probably increase the directional stability but also hinder skid ("dérapage") (if such a thing is desirable [for the fun])
    Things are always more complicated then they seem at first look.

    "we might have to build one more, so each of the boys can have his own. " What does your girl think about that?
    She might prefer a sailing dinghy which is also a lot of fun but of a different nature. And she could proudly take in tow the 21st century boy when he runs out of gas.

    1. Hi Sylvain.

      The lath/compass issue is interesting. The plans are not so detailed as to describe how they are done, but given that they don't inform about a radius they might use the lath method. If I use a lath to make a curve, the final outcome ill greatly depend on the thickness of the lath and also the wood used to make it. A 0.5 x 0.5 cm lath will make a different curve than a 1x1.
      In real life I don't think it is so important, because the plan describe that you need to adjust and trim the parts before mounting the top piece of plywood anyway.

      In one of the build videos that I watched, they made the fore center strut triangular from the start, then after bending the bow, they adapted it to fit the curve that was on that particular boat.
      I think that there will probably be some degree of fitting no matter how I make the individual parts. But I would prefer to keep the fitting to a minimum, cause that will mean that I have worked fairly accurate :-)

      From what I have been told (and read), the engine should sit so high up, that the propeller is just below the bottom of the boat (which is also what Paul Hawkinson wrote in his comment).
      Another thing to experiment with is the angle of the motor relative to the hull. I can't remember the angle of the motor mounting plate, but there is also 3 or 4 settings on the motor itself.
      I am pretty sure that on such a small and short hull, there will be a huge difference depending on the weight of the driver and what angle will be the most efficient.
      I have thought about making a small seat instead of the original "kneel on the bottom" layout. That will mean that the center of gravity is fairly stable, and might make it easier to trim the boat.
      I am not sure if the small outboard motor we have at home has got a long leg or a short leg. But if we have to get another one at some point, I will try to go for a short legged version. to keep the motor as low as possible.

      There is a very shallow keel along the bottom to prevent skidding while cornering, so I don't think that a rudder will be needed. In fact I think that a rudder might not be a good solution when there is an outboard motor. Unless I locked that one in place.

      Laura will not be at home this summer vacation, as she has bought Interrail tickets with a couple of her friends, so she'll be busy seeing Europe from a train.
      Another thing is that I really doubt that she will be interested in a boat. Now if I was able to make a spinet or a piano it would be a different case!
      Besides, if there is enough wind for a small sailing dingy, I think that there will be too large waves for the hydroplanes. I imagine that they are not very seaworthy!

      I myself would like to make a nice little clinker / lapstrake tender, but that will definitely take more time than making a couple of those hydroplanes.


  9. I just re checked the drawing, and it seems as though the underside of the fore cross piece is not a true curve after all.
    It is a V-bottom, but instead each flat of the V is made 3/8" deeper on the middle. So that can either be done with a lath or or a compass. But I think that I''ll go for a lath on that one cause that will be the easiest.
    The drawing is page 2 of the 5 pdf files on the site.

  10. I'm really interested in this project. I bet it would be fun zipping along in the harbor here, but the big yachts probably wouldn't be as amused.

    1. Well, as long as you don't steer directly in front of them they should be OK.
      Just remember to check the local regulations. Most ports have speed limits inside the harbour.
      Off course there is also a certain element of fun in doing something with the risk of being chased by some officials, but I would hate to know that you got into trouble with Guardia Civil because of my advice to build a Mini Max and play Evil Knievel :-)