Concept to basic line drawings - 40 foot power boat

I mentioned earlier in this thread that I would fix a couple of things in my drawings, ie, sole below water, etc.

These are the results. I did a revised profile trying to show the huge notched transom with the surface drives shown. Also tried to show where I thought the engines would go.

I added a revised plan view now with a padded sun pad in the rear over the notch. Also tried to show engine boxes like you might see in the Bertram 31's so as to allow the balance of the sole in the helm area to remain lower.

And I added a cutaway plan view again to show where I thought the engines might go, the revised sun pad arrangment and even the cabin layout (although I would still not install a head in mine, others seem to prefer it so I showed it).

Any better? Does it answer any questions or raise new ones?

Thanks,
Dennis

 

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The surface drive is too low. Also the angle should be no more than 4 degrees to the waterline.

 

Gonzo,

The angle of the surface drives may be a moot point due to their cost. If I have to notch the transom to reduce the LCG and add a sun pad over the area, I might as well just go with stearn drives. I was trying to keep the cockpit large and clear, but this may be better. I may revise my drawings again.

I think my next attempt at drawing will be bow and stearn views. And I may try to add some dimensions and scale to the original drawings.

Thanks for the comment.

Dennis

 

You can run sterndrives with a jackshaft for more cockpit space. The shaft will be about 11" from the edge of the bottom under the center of the drive.

 

Yes, stearn drives with jack shafts would keep my cockpit larger and clear, but would that help with the problem Doug mentioned with the weight of the engines being possibly too far forward?

I envisioned this as a planning hull and due to the area that I do 99% of my boating I want the wake to be as minimal as possible at cruise speeds. I see a lot of other boats in the 35-40' range coming up the river at 20 mph with their bow pointed towards the sky and a 5' tall wake behind them. They don't even slow for other smaller boats. I wouldn't want to be one of "them".

Dennis

 

If the displacement curve corresponds to the load it should be not problem. Those boats with the nose pointing up are, in my opinion, lousy designs. They are made to maximize volume for a given length. Since you are looking at a swim platform, one way to help slow speed behavior, is to use the platform as a double end stern. Instead of draggin water it makes a streamlined exit. As the boat lifts up on plane, the water doesn't touch it.

 

Designing a boat is like juggling six balls at once, it goes fine if they stay in line, you've got trouble if one doesn't fit.

A forward LCG is avoided because it can cause dynamic instability. As you observed, it is sometimes avoided to excess.

On your sketch divide the waterline into ten equal divisions, number them 0 to 10 from the stem to the transom. for a 25-35 knot boat the engines need to be between station 6 and 7. The fuel tanks also need to be around this spot, say 5.5 to 8. You need this machinery weight aft because it has to balance the weight of accommodation forward.

All the best, Tad

 

I'll add the station lines and see where it comes out. When you say between stations 6 and 7, how is this affected by a transom notched to the degree I show? Is there a formula to determine how much a shorter running bottom surface will affect the LCG?
Once this is done, how does one estimate the total weight of a proposed design? There are huge weight differences between power plants, drives etc.
How do boats like the 60's thru early 80's Bertram 31's get away with having their plants so far forward and then piling on flybridges and such? Is it their lower top speed requirement? Are they planning or semi-planning hulls? They seem like their LCG would be forward of amidship.
Dennis

 

On the subject of drives, there are many possible options. If you go for sterndrives, however, I think you'll be restricted to using those made by Konrad as Volvo and Mercruiser don't produce recreational drives capable of taking the power from the big diesels that I assume you'll be installing.
The DBD Marine drive is an interesting option. They are made here in Oz, are cast from stainless and have very few moving parts. In spite of appearances, they aren't a surface piercing drive - they use conventional sterndrive props. Their largest model is capable of running a commercial duty 500hp diesel. I've seen them at local boat shows a few times, but only ever installed on smaller (ski) boats. Whether they have much in the way of distributors in the states I don't know. They would certainly be cheaper than Arnesons - and wouldn't require the complexity of your notched transom.....

 

I had thought the surface drives would be more efficient then a regular stearn drive or a inboard but I had no idea that they cost as much as they do. The Arneson with a drop box is close to 20K. Each.
The notch in the transom was not so much for the drives, but because it was mentioned that it looked like the weight was too far forward and this was just an idea I had to move the LCG aft. I don't know if would work, just a thought. If it wouldn't work, I don't need the notch.

As I stated in my last post, I'm going to add some station lines to my earlier drawing to give it some dimensions. In looking at some other drawings I noticed that when they start at the stem and go to the stearn, they don't start right at the tip of the bow, nor the water line. Where should the station lines start and finish along the profile?

 

Station "0" should pass through the intersection of the bow and the water line.

Station "10" should pass through the intersection of the transom and the water line.

Surface drives are more efficient than either shafts or stern drives at speeds over about 35 knots. Stern drives tend to be more efficient than shafts. However at 40K, you'll need to be doing a LOT of miles to justify the extra expense.....
There are other, less costly, surface drives you could look at: Qspd, Seafury, Trimax etc etc
At the end of the day, however, if you're not going very far and don't care about wringing an extra couple of knots of top speed out of your boat, then conventional shafts are by far the most cost effective and reliable option (so long as you don't want to beach the boat)

 

Willallison,
I added the station lines using the water line with the boat at rest and the engines are between stations 5 and 6 slightly in to 6-7.
If I made the bottom of the running surface due to the notch station 10 and re-draw the lines, the engines would be between 6 and 7 slightly into 7-8.
Which is correct?
Does it matter that the drives are furthur back?

I don't travel far in my boating trips. If I put three hours on the engines in a day trip it would be a long one. I'm re-thinking not only the drives but the diesels. My current boat is a 1989 I bought new and has 400 hours on it. At this rate I would never offset the increased cost of the diesel engines with fuel savings. I could use less expensive and lighter gas engines, use stearn drives with a jack-shaft arrangement, and probably never wear them out.

I'll post the drawings with the station lines tonight.
Thanks,
Dennis

 

Dennis,

Take a look at the section on planning hulls in Priciples of yacht design by Larsson and Eliasson. Also try to get a copy of Technical and Research Bulletin 1-23, How to use SNAME small craft sheets for design and resistance prediction. SNAME.org
I like the eveolution of your sketches, keep up the good work!

 

Station lines are normally added to the extreme ends of the at rest waterline.
(I think) that when Tad was talking about engine placement he was referring to on board weight, rather than the engines themselves. A pair of big diesels weigh a great deal, so their placement is important in the overall weight placement equation. Fuel and water tanks are located as close as possible to the LCB so that it doesn't matter if they're full or empty.
The biggest problem I see with your notched transom is that it reduces the surface area of the bottom. As Tom Lathrop will happily point out to you (and I think may already have done so earlier in the thread), if the mass per unit area of the bottom is too high then the boat will require enormous amounts of power to get on the plane and conversely will tend to drop off the plane at relatively high speeds. It's true that this increased area will also increase drag, but given that we aren't talking about a very high speed craft, I think that low speed perfomance is way more important.
I agree that for the number of hours you notch up then petrol engines will probably make more sense. Personally, I'd go for a pair of fuel injected petrol inboards driving through conventional shafts or perhaps through stern drives and jack shafts.
Either way, you'll probably have to move them aft a little or boat won't float level and could be quite a handfull at speed. With nothing but open cockpit down the back end and all your machinery and accomodation located fwd it's inevitable.