34' + x 10.5' - Design Questions (Twin, Triple or Quad)

What's the best way to determine the "ideal" transom angle for a center console sportfish?

If the hull shape has some convex curves, is the deadrise measure to the tangent of the curve or from inside (keel) to outside (beam)? I assume that the deadrise if from keel to beam at the transom.

How about the transom height? Should it correspond to the shaft length engine? For example, a 30" engine would be mounted to a 30" transom. In my case, the transom height is defined as the vertical distance from the keel to the top of the motor pan. True?

A triple engine setup usually has a 25/30/25-inch setup. Does a quad engine setup have 25/30/30/25 inch shafts?

How much offset should be allowed for between engine centers on a 24.5 degree deadrise boat? I recall a tech rep with the engine companies stating that the range is between 26-30 inches from engine center to engine center. Should this remain the same regardless if the engines are multiple 150hp outboards or multiple 300's?

Since the boat will be made from fiberglass and I'm attempting to maximize the cockpit area, I'll be using very vertical sides. The next question is how much TRANSOM draft angle should I allow? I recall using minimum 2-degrees draft, however, would like some input from those who have built plenty of molds. I realize that vertical surface could not have any shape (concave or convex) otherwise the 1-piece mold could not be used. The part would get locked into place following layup. How little draft can I get away with and still pull the part?

 

I vote you go to big boat shows with a camera and shoot any boat you like. Save time money and quitting on a design that never had a chance. I have done that at least 1 time. Copy a winner with a proven track record.

 

Ouch, that hurt. No need to be testy. Didn't your parents ever teach you "if you don't have anything nice to say, then don't say anything at all"?

 

Don't take it personally, rdeputy. My interpretation of what Cyclops said is that the answers to your questions will become clearer if you go poking around the undersides of boats that you know have done well. If all boats in the class have a similar angle to the transom, that probably means that angle works well. (Vertical transoms, btw, don't usually work too well- you need that slight angle both to keep water from coming in when backing down, and to let you tuck the OBs in when you first come up to plane.) Taking a tape measure to the mounting brackets of outboards that are already mounted will be a good indication of what spacings to use. In short, take a good look at what works, then see what you want to improve about it. (Also, about your molds- Vertical sides are hell to pull from a 1pc mold; you might get a nicer hull from a good 2pc mold.)

 

Good question.

I measured mine for you (twin 115 Honda 4 stroke) and it is 35 degrees if I did everything right!

(My method was to lower jockey wheel on trailer at front until deck was level by spirit level. Then place spirit level against transom at top between OB's and take a couple measurements with a tape measure at 300 & 400 Mm's down from top of transom between transom and spirit level when plumb. I drew these out on newspaper and measured the angle from the transom to plumb starting from the top - with a protractor and got ~ 35 degrees or thereabouts).

I seem to recall that the outboard owners manual has rigging isntructions which specify what transom angle they are sposed to be bolted too. (And spacing apart for twin rigs etc).

Of course transom angle can be altered with the use of transom wedges, if you need more transom angle.

As to height - length of leg - If you have a deep V and the spacings apart on triples or quad installation is sufficient - then you MIGHT want a 25 & 30 inch set of leg / legs for the outside / inside engines respectively, BUT - you'll still have to mess with ideal vent plate heights by moving the engines either up or down on their bolt holes & test each time with various prop diameters and pitches to get an "optimum setting" and this testing by trial and error could take "forever"..

Probably bye the time you got it right - the engines will be wore out and you'll need to repower and start again!

Probably your "best option" is to fit all 3 (or 4) OB's onto their own respective jackplates (Bobs Machine Shop in Floriduhhh) so you can raise or lower any individual OB "on the run" and monitor any improvement via fuel flow gauges - and gps speed as you go - tweaking whichever engine you think ain't pulling it's weight - individually...and then monitoring any result.

Often a good indicator - is the prop wash (rooster tail part) of each engine, when you look back!

If they are all intersecting the water level behind you at the same distance back - then likely you got it set right. Also they should converge toward a central point aft of the vessel (determined bye the amount of toe in you give them on their steering tie bars). Setting toe in's another grey area, it varies from hull to hull and engine installation to engine installation.

Believe it or not... - the best way to set toe in (this will draw some raised eyebrows) - is to idle along at just above idle speed (800+ rpms) or thereabouts.

Assuming you have say triples, then the centre engine can be fixed by it's tie bar, and you steer straight ahead. Either side engine tie bar are unbolted, and the engine allowed to "find it's own path of least resistance".
Then the tie bar is wound in or out to meet this position and re positioned and re tightened at that point. Basically - the water flow over the gear case / skeg and past the prop - pulls the OB to it's path of least resistance thru the water..which is where you will get best top end and least drag once fixed in that position.

You do the same with the other side.

Hope that helps some.

Cheers!

 

Rdeputy. Marshmat read my remarks to you perfectly. Pick a proven boat. Then personalize it. I am a know it all type, sometimes. I STILL pick a boat plan I love and THEN modify it if needed.