I have been looking for right project for about 2-3 years and I think I found it.
Right now I have question about closed transome, is it possible ? ( I want to have more space at aft )
I would like to have outboards setup like this :
- 3 outboards on Porta bracket ( hydraulic bracket which can lift up and down outboards ) with set back of 26", how will this affect to CG ?
- two 115 hp 20" Evinrudes on each side and one 25 hp 25" kicker for trolling in the middle (with separate tanks and controls)
And please tell me something about what is predicted behavior Gerat A. at displacement speeds, do you maybe have projection of knots vs hp ?
Hi Jadranko,
I will try to answer your questions one at a time. Yes, a closed transom is fine (see attached) and the details already completed. This was for a sterndrive option, but should work for a bracket as you describe. My only comment on the horsepower choices is that they seem a bit high. This boat is a lighter than your typical fiberglass or aluminum boat and requires a lot less horsepower (and gets a lot better mileage). Unless you are really going to load the boat down, 175 hp (total) would be fine. For twins, you could use twin 90's, or round up to twin 100's or 115's primarily. The larger options would primarily be for higher speed, possibly planing, if you lost one motor and had to return on one... but you certainly don't need that much horsepower for normal operation. The same applies to the trolling motor. A 15-hp trolling motor would work fine, but rounding up to a 25 hp would be OK. If trolling motors are too big, then you risk not being able to troll slowly enough unless you buy an extra-low pitch and/or smaller diameter prop for it. Here's a link to the hydrodynamics (displacement and planing, speeds and stability) analysis that I performed on this design:
http://www.glacierboats.com/node/36 Note that the analysis above is for a perfectly smooth hull with no parasitic drag, e.g. drag caused by skegs, strakes, trim tabs, motors etcetera ...the predicted speeds are about 10% faster than what you'd realize in the real world.
As for mounting your motors behind the transom on a 26" porta bracket, it's possible but I'd want to get more information from you on your intended accommodations, fuel requirements, and what not so I could do a careful CG analysis prior to your committing to your plans. I think that the worst case would be giving up the aft belly tank and moving fuel forward either into saddle tanks under the port and starboard sheer decks or a pair of box tanks hiding in a settee or other location (just inside the aft pilot house bulkhead) to counter the weight of the motors being moved aft. Your selection of 230 total horsepower leads me to assume that you are leaning towards a 28' (longer rather than shorter) boat? If so, then it'll be more resilient to shifting CGs than say a 25' Great Alaskan would be. See Inboard Construction Profile (attached).
Brian
