RMS Some More

What's the diff?

In the hull proper there isn't much difference. In the original version the bow transom was curved at the top and also curved athwartships. Working out the build for this little quirk was difficult and it may be beyond the experience of the typical backyard builder so I made the new transom flat.

In the original design the keel was a flat plate with a the keel  attached to it. This came out of a design I was toying with much earlier. Here is a picture of a model of that design. I have long since lost the actual drawings during several moves but the model remains. You can see in the picture of the bottom the space for the keel piece which is a tapered, curved and beveled 2x12. Again probably not the best design for a backyard builder. It took me quite a while to get the model right.

I am a great believer in models. When I did the design for the WoodenBoat competition I made a half model to make sure the design would come together off the paper.

In the next  picture you can see the keel piece.

The change to the keel, which adds a box to the keel was to get the flow right to the new motor which is electric versus gasoline and we'll delve further into that as we develop the design. I don't think that the box keel will be that difficult to build as long as the build method is ply or plank on frame versus stitch and glue.

My instructor at Westlawn commented that this would be a wet boat because the sides are almost up and down at the bow, he obviously hadn't spent any time looking at Phil Bolgers designs for small boats.

RMS

The First Sampan Skiff

In 1990 I was deeply involved in the Westlawn process and WoodenBoat, an excellent publication, ran a design contest for a 15' boat that would be equally at home being rowed, sailed or running under a small motor. As part of my course work I entered that contest Here is part of the lines drawing, I can't show you it all because it was all done by hand to a scale 1”=1'.

 The quality of the picture is very poor because it is a scan of a photocopy of a 16 year old drawing ( I never throw anything away). However I show it to you so you can see the detail that is required.

I have taken that design and redone it in my CAD programme, changed the hull a little bit and altered the lateral resistance quite a bit. Here are the new lines.

The original design had a fore and aft box seat for rowing with a hinged top into which you could stow the spars, sail, leeboard and oars. It had a small, 2hp, outboard that just cleared the tiller when tipped up, was clear of the keel but did not clear the water. I think you can all see the problems all this would create.

Over the next few weeks we'll look at the changes made and discuss why I made them and work this up to a completed design. I'm doing this is smaller chunks so that I can actually get a post out at more regular intervals.

Round Bottom Boats

Round Bottom Boats

In the last post I said that your client had changed his mind, what I intended to do was to modify the power dory to take an outboard. However whilst I have been taking a break from posting, primarily because I actually have to create the design from scratch, I was contacted by a follower who wanted more rounded boats, so here we go,

Several years ago I designed a rowing boat which I called Sally Blank, (white hall=salle blanche= Sally Blank, I told you I liked puns) she looked like this,

 I called her the poor man's Whitehall skiff. However she was hard chined, here are the lines.

So to satisfy the request for a round chined boat, I redesigned her. The process is much like that described here and here. However in order to create a fair hull more lines are needed. Here are the almost finished lines of the new Sally Blank.

You can see that there are far more defining lines than in the drawing of the hard chine version. In the first version you have only one waterline, which you don't even draw in, here you have four which must be drawn in. Those lines give you the hull shape looking at it from below.

Then you have buttock lines, those are the lines in the section drawing parallel to the centerline. When transferred to the profile they give the hull shape from the side.

There are some extraneous lines to be removed and a last set of lines drawn in.

If you carefully compare the hard chine version and the round chine version you'll notice that the chine line and the keel line from the hard chine version are shown in the new version, that's because I used those lines to establish the new hull shape and the location of the chine and curve of the bilge You'll also notice that the stem is still straight which it can't be in the new boat. So we'll remove the extraneous lines, alter the stem and draw in a keel.

You'll notice two new lines in the sections drawing, those are a diagonal which will give us an idea of the flow around the hull. If we've done this right that diagonal, which will show as red line on the profile, will also be a fair curve.

C'est bon, n'es pas (yes I know I fractured the language).

So there you have it from a hard chine row boat to a round chine skiff. The work is not difficult, just tedious however, as I said here, using a computer speeds up the process.

Maybe we'll start something more ambitious next time.

The short strokes

Finishing up (this one)

I have been remiss, my other work has dragged me away from this and put me behind schedule however I'm back at it and so we'll finish up.

The last thing to be done with this design is to add the finishing touches, to add to each of the drawings those things which are present in other drawings and which are relevant to a particular drawing.

So on the inboard profile we drew in the engine bearers those need to be transferred to the construction profile and to the construction plan.

To the inboard plan I added chocks at the stern for berthing lines, those need to be shown in other drawings.

Lastly it is usual to draw one or two typical construction sections and in larger designs furniture detail in this case we'll just do the mid section

You can add specifications directly to the drawings or do as Bolger did, number all the parts and draw up a specification list.

When your drawings are done you can either put them all on one big sheet or do up separate drawing sheets for each of the views.

Here' the sheet for one of my designs, Vole.

In this case I wrote a build document that specified all the parts and sizes and the building process.

Next time we'll throw a spanner in the works and have the client change his mind about what he wants.

It's all about profiling

Outboard profile

The out board profile is what anyone looking at your vessel from the side will see. One of the perils of drawing outboard profiles by hand is the perspective effect caused by you looking at the profile with its top edge away from you. What appears to be a neat boat on the drawing board suddenly doesn't look all that swell when pasted up on the wall. The advantage of computer drafting is that you are already looking at the boat as it would appear on the wall.

When you are drawing the outboard profile you should include everything that you, as the designer, planned to have fitted on the boat, the builder/owner may have other ideas. So be it.

So we start with the outline of the hull as we drew it in the construction drawing. And remove any extraneous lines.

So we need to add a house. If you recall the original design called for a place to get out of the wet and an open cockpit. The design was also for up to three adults so two can be out of the wet and the other can steer. So the house should be at least half of the waterline length. And a way for those inside to see outside, a port perhaps and or a hatch. Now a comfortable seat needs to be about 15 or 16 inches high and with the same depth and minimum head room is 38 inches so the cuddy top should be some 4'6“ above the midship frame. A hatch needs to be a minimum of 2 feet square. The port can be any size or shape but should fit with the overall design. I always like a small foredeck, not that you would want to stand on it, but it gives a place for a substantial samson post forward and a small storage locker below.

So all that would look like this.

The cuddy would be open at the back for air in the cuddy and also to provide air for the engine which is air cooled. We could also decide upon a rudder at this stage.

The rudder could be transom hung or be under the boat on a rudder shaft. Because this is a small slow boat I think that a transom hung rudder would be better and fits with the type of boat.

OK you say but what about the propeller and engine. Well in this profile you can't see the engine but you're right about the propeller. The problem is you can't show the propeller until you decide the angle of the shaft and to do that you need to decide where the engine is going to go. Which brings us to the inside profile.

I lied when I said in the last post that we would do the deck plan, we really need to establish where the engine will go and what the shaft angle will be because then we can finish the outboard profile and the construction drawings for the keel.

Inboard Profile

We start with the almost completed construction profile, see why this is easier on a computer.

Then we add the cuddy that we drew on the outboard profile.The port, samson post, fore deck and some seating at the transom and in the cuddy.

Now for the engine. We have the specifications for our engine. It is 15.4 inches long, 12.6 inches wide and 13.6 inches high, the shaft height above base is 4.17 inches to the centre of the shaft. The propeller that we have is 8 inches in diameter so once we draw that in with the shaft exiting from the back edge of the keel and the propeller tip 2 inches away from the bottom of the boat, to stop thumping, we can establish a shaft angle of 5 degrees which will not interfere with the lubrication of the engine.

There is a little bit of a problem, the prop spins just a little below the keel which, if the boat takes ground, will cause damage to the prop so we'll put a skeg on the keel to protect the prop.

Now that we have established the shaft angle we can extend that back into the boat and determine where the engine will sit and draw in the engine bearers. But we have a problem, the optimal RPM for the prop is 600 RPM the engine runs optimally at 3600 RPM so we need reduction gearing. There are two ways to accomplish this, an actual reduction gear set up or v-pulleys. Given that this project is driven by cost, V-pulleys it is. A 2” steel pulley on the engine output shaft and an 8” pulley on the propellor shaft.

All of which means that the engine must be several inches above the shaft centreline probably, about a foot. There are calculators for this at http://www.gizmology.net/pulleysbelts.htm. For the purposes of this exercise we're going to place the two shafts 12” apart. Given that distance we get this,

The square box is the engine, the area under it is the engine mount, the beds should cover at least three frames to reduce vibration and strain on the hull. Covering the shaft coming into the boat is the shaft log. The concentric circles on the engine are the exhaust. We can extend the exhaust to show on the outboard profile and draw it in.

And there is the almost complete outboard profile and the almost complete inboard profile.

Outboard Profile

Inboard profile

Next time we'll finish both these drawings and move on.

Construction Redux

Construction Redux

When we finished last we had drawn the construction profile to the gunwale and added an outline of the keel. This time we'll look at the construction plan view.

I transferred all the construction details from our previous drawing to this one.

You will notice that I've only drawn in half, why draw two things that are exactly the same? Anyway the other side is for the deck plan once we decide what sort and size of house we want.

Next we need to deal with the transom and we should draw that in detail and then transfer it to the construction plan, Once it's drawn in detail you can later add specifications. To draw you take the height of the transom off the lines plan in profile and the width at the top and bottom from the sections.

I've added a sculling notch, framing around the edges and a centreline backing block for the rudder attachment. When drawing the transom , or any other detail view for that matter you should show two views as a minimum.

The bevels cannot be taken off the lines plan as the transom is sloped 27 degrees and the sides slope outwards making a compound bevel so draw the transom as if it was square and let the builder do the bevel in situ. However having said that you can draw in the bottom to transom bevel as that is not compound and the bevel can be taken directly from the lines in profile.

Once all that is done then the transom can be added to the construction plan in plan and profile.

Now comes the question of floorboards. Are you going to have any at all? If not then it makes no sense to have a keelson as that becomes a tripping hazard, not good in a small boat. I lean toward full length floor boards which are removable for cleaning then a keelson makes sense.

Next time we'll drawn up the out board profile and then the deck plan and return to the construction plan to add those features

Going Whole Hog

Going Whole Hog

Last week we ended on the thought that maybe we should have a second stringer, I though some more and put it in.

We are going to draw in the hog, the hog sits atop the bottom planking and is fastened directly to the keel through the planking. It's partner is the keelson which sits atop the frames and is through fastened to the keel through the frames and hog. It is not strictly necessary in this small a vessel but we will discuss it later when we decide on decking.

The hog is let into the stem knee and the transom knee in a similar manner.

Then I did some tidying up. When you look at the construction drawing in profile you're actually not looking square on, in this boat, to the stringers as the side has some flare to it. In actual fact you would see the bottom edge and bottom of the stringers and wales so I've drawn them in. Similarly the frame ends would stick up beyond the top of the inwale.

Then we'll add the bottom planking, the scantling rules suggest 5/8 planking however plywood doesn't come in 5/8” and 3/4” ply would be too heavy so 1/2” will have to do. 3/4” ply weighs about 75 lbs and 1/2” only 50 lbs.

We'll draw in the keel next but leave it in profile and develop a detailed drawing of its construction once we know the shaft diameter and angle.

Keel

The last bit for today is to draw in the cap rail.

Next time we'll look at transferring the construction in profile to the plan view.