Wednesday, June 26, 2013

Building a Classic Wooden Motor Boat - Part 3

This is part 3 of a chronicle on building a wooden motor boat or launch. 

In the previous chapters Walter the boatsman built the hull, and then we went with the kids to pick it up.

Now the Boat sits at the technical department of a large boat sales company here in Santiago, where they are fitting the engine, and instruments and controls.

We went over there with my wife to look at it a few weeks ago, to see how the work is progressing and take them the instruments that will go on the dashboard.

Engine Fitting

The engine I selected is a 4.0 liter V6 Mercucruisier Sterndrive, with the Alpha One Drive. I chose the carburated version over the fuel injected, since i can do carb maintenance and tune ups myself, and is actually something i enjoy very much.

I didnt want to go with the 5.0 liter V8, because of noise, fuel consumtion, and the extra cost. Plus I dont think i'll be needing that much power, since the objetive is to have pleasura rides, and not a super fast boat.

To be able to fit the engine, they had to fit two pieces of 14 mm thick marine plywood inside and out, making a sanwich with the stern, this is for both having perfect smooth and parallel faces, where the water seals of the sterndrive will go. You dont want water entering Through this big opening under the waterline.

The marine plywood pieces go glued and then screwed, and the screws covered with epoxy putty. In order for the gluing process to work well you have to wood that has 12% or less moisture content (MC), and upon arrival to the shop the boat had 15% MC so we had to work 3 weeks for the MC level to go down before the work could get started.

Now the wood is below 12% MC and as you can see in the pictures the opening for the drive is already cut in the stern, in a perfect cut that almost lloks like they extracted a violin out of it.

The aft part of the keel had to be shaved considerably (only on cm high remains on the outside) in order to reduce the amount of turbulence that reaches the propeller from under the hull. But in fact i will have to remove more of it in order to reduce hull drag and get the boat up to planning speed. More on this later.

We also took a nice set of 6 vintage looking white dial instruments for the dashboard including all the common stuff you will need to monitor. I still have to find the adequate control box with the speed and fore-aft levers, as I already got an old style wood rim steering wheel with aluminum spokes, same as you'd find in a 50's british sports car.

Yesterday they called me from the shop telling me they need to enlarge the engine doors that go on the deck towards the transom, since the engine fits very tight and routine maintenance such as cheking air filter of cleaning carbs would be a pain, and i want that task to be comfortable since i'll be the one performing it.
It could be left at it is but they woud have to shave a lot off one of the crossmembers under the deck and this could leave a weak spot in that part. So we are going to have a larger engine bay to work on.

Hull improvements

Meanwhile i've been reading a lot about hull design and hull structure, the enginnering or intellectual approach to doing things as one friends puts it. In doing so ive discovered that structural dimensions are fine and exceed what is needed, which is good news. Also ive discovered that the hull shape in general is fine and should lead to a pleasant ride, but I can make a few improvements to have a more sporty or agile cruising (read make sure the thing will go moderately fast).

This hull is originally a displacement hull, if i want it go fast I need to turn it into a semidisplacement or better yet a planning hull. To to this I need to approximate the aft section to a flatter and squarer form, since round forms dont plane, they get sucked into the water.

Since the aft bottom has a moderate V (11 degrees) I shoud add some running strakes in th emid section, to help give it more upward lift as the bow rises and it tries to climb its own wave. Running strakes are like water skies fitted to the bottom of the hull, with the surface horizontal (parallel to water) instead of having the slope of the V.
This will make a wing-like effect and create a plain vertical lift force, as opposed to the inlcined one the V surface would create, thus more effciently raising the boat and helping it get out of the water to get into a plane.
They only run in the center section, since at speed they create more drag than a plain bottom, so whe the boat is already planning in the aft section of the hull you dont want in the way anymore.

The other modification I need to make it modify the round chine (where bottom meets sides) into a hard chine. This is because water tends to climb aling the sides of the boat with a round chine, creating more drag which eats your power. But water cant clinb a sharp 90 degree turn (a right angle), which is what a hard chine is, it just gets separated of the hull, expelled sideways, letting your boat run free of that drag.
This is the reason why speedboats have vertican or even negatine angle transoms, with sharp angles, instead of the round aft section found on displacements hulls - as a sailboat (which under power only lowers the back end but never gets out of the water).

After I get the hull back from the engine and control fitting, I need to make these changes to the hull prior to the sanding, and varnishing (or painting below the waterline).

No comments: