Having your cake and eating it too.

[TnD]

First off, Hello to everyone.

I'm looking far into the future for a boat that I would like to work toward eventually.
Here are the design elements...Aluminum stressed hull design, 36 loa, 11 beam, planning hull, Pacific NorthWest workboat style (forward sloped cabin front), 18 degree deadrise at the transom, slighltly inverted chine, and lifting strakes with about 8 ft of open deck aft as a lounging/fishing platform. Displacement in the 10 ton range.
For propulsion I'm thinking of twin Cummins QSB5.9-355 INT through TwinDisc to Arneson SPD's. According to Arneson's calculator, this would give a top speed in the 41-45 range, they don't specify knots or MPH.
Questions are...is this speed realistic, and secondly, and more importantly...what would intermediate speed performance, (i.e. getting on step...what speed??) be like assuming that the person trimming the drives knows how to get Arneson's to perform.

[RANCHI OTTO]

Try with this..

KR = (BHP)^0.551/(Displ.)^0.476

where BHP metric hp
where Displ (t)

[TnD]

Thanks for responding Ranchi, although I feel I am misinterpreting your formula as I am getting a number of KR=81 which is way off the top of the graph in the Excel spreadsheet. I think I need to back up and do some more research to give myself a better theoretical knowledge to work from so I can properly put equations to use.

[RANCHI OTTO]

(2 x 355)^0.551 = 37.24
(10)^0.476 = 2.99

KR = 12.44

Speed = 36 knots abt.

The speed indicated by Arneson is in my opinion too optimistic.

I've designed a military boat with D = 11 t and 2 x 430 hp with surfaxe propellers and speed of + 40 knots!
This was a very good results...

[TnD]

I thought Arneson was quite optimistic as well... I was recently on an aluminum boat of similar dimensions to the one I've described, perhaps 33 ft and slightly less in displacement, running jet drives, and it topped out at 36 mph.

I must admit that I'm feeling a little thick in the head, as I still can't seem to get your formula to work for me...what function are you indicating with the ^??

[RANCHI OTTO]

The KR diagram is to be used only with piercing propellers, for waterjet propulsion with a KR = 12.44 the corresponding speed would be 32 knots abt.
This figure corresponds very closed to yours.

The "^"means googol (exponent)

2^4 means 2 x 2 x 2 x 2 and (2 x 335)^0.551 means 770 exp.0.551

[TnD]

Ahh, I see. That clears it up significantly, thanks for your patience.

[RANCHI OTTO]

You are welcome!

[Willallison]

18 degrees transom deadrise would seem a little on the low side for a vessel intended for fast rough water work - 23 to 26 degrees would be more normal, though of course you would sacrifice a little top speed in calm conditions...

[TnD]

Been a while since I have followed up on this thread I started.....

Williamison,...Deadrise is an area that I need to look at closely, as well as the rate of change in angle as you head toward the bow.
Unfortunately I have no formal training in design and what I learned while working at an aluminum boat manufacturer was learned from people who were self trained as well. I have heard seen a variety of setups that all seemed to work. However I would like to have some sound design concepts or guidelines to work from rather than just winging it, especially with the price of metal these days. I also want to be able to confidently sell a boat that I have designed and built, with no feelings of apprehension. Skills to do the construction are not a problem, its the design issues that I need to overcome.

I tend to spend a lot of time under boats at dealers with an angle gauge

[Rik]

Depending, and this means a lot, depending upon your hull design, you should reach 33 knots at worst with a 39 knot high varying upon design, balance and setup.

Mercury Marine made a formula which is very basic but gives a quick reality check based solely upon displacement and power.

The Square Root of (Total Shaft Horsepower / Weight ) x Constant = Speed