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Old 04 June 2003, 13:06   #1
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Country: USA
Town: Portland
Make: Polaris
Length: 5
Engine: 90hp- 2stroke!
Join Date: Apr 2002
Posts: 15
Dive boat RIB size for 90hp Merc???

Hey all,

I have a 2003 90hp 2 Stroke Mercury. (yes 2 stroke, gas is cheap here in Oregon) I purchased this motor for my current RIB with the intention of putting it on a larger boat. Well, the time has come to get a new boat. I have a manufacturer that will custom build the boat for me, and I am looking for suggestions on the actual size to build.

It will be an aluminum hull, with a gill bracket for the motor. This is also know as a motor pod, or transom extension bracket, that sets the motor about 2 feet back from the transom.

I can specify the beam, length, etc. The primary purpose of this boat is for SCUBA diving. I will have a very small center console, but the rest of the boat will be open. I will typically have 4 people and 8 tanks, and of course the weight belts, etc.

For the 17'6" model, the boat weight alone is about 800 lbs. (Sorry for the non metric system measurements!)

As this is going to be custom built, I really could use the feedback that will help me work with the builder to get the right boat. This boat is not really going to be used for real rough water. I plan to have a fairly wide beam on the boat.

Please let me know any and all suggestions.

Thanks
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Old 05 March 2004, 01:53   #2
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Country: USA
Town: Chula Vista, CA
Boat name: T.A.P.O.A.F.O.M.
Make: Ribcraft
Length: 5m +
Engine: Suzuki DF140
Join Date: Feb 2004
Posts: 30
Engine Pod or Transom Extension

Here is an image from the Polaris website showing an engine pod. I wonder how this would affect the stability of a Ribcraft. With this setup you could put a bench seat against the transom and have extra floor space for divers moving about. I still have a few weeks before my boat hull starts getting built and I am considering this option. I guess this is not a very popular option as I see few posts on it.



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Old 05 March 2004, 05:31   #3
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Country: UK - England
Town: Salcombe, Devon, UK
Boat name: BananaShark
Make: BananaShark
Length: 10m +
Engine: 2xYanmar 260 diesels
Join Date: Mar 2003
Posts: 4,225
Re: Engine Pod or Transom Extension

Quote:
Originally posted by Manny
Here is an image from the Polaris website showing an engine pod. I wonder how this would affect the stability of a Ribcraft. With this setup you could put a bench seat against the transom and have extra floor space for divers moving about. I still have a few weeks before my boat hull starts getting built and I am considering this option. I guess this is not a very popular option as I see few posts on it.



Looking at the photograph it appear that the extension is also a hull extension so adding 2 feet or so to the running length of the boat - this would effectively make a 20 foot boat a 22 footer making it a much better rough water boat than the one without an extension.

The problem I could see with this is that the centre of gravity would effectively move forward which would not be so good as far as performance is concerned.

Having said all of that I assume Polaris have done thier homework - is it mostly to support a heavy twin rig setup? That might then move the C of G back again?
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 05 March 2004, 09:46   #4
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Country: Ireland
Town: Cork
Boat name: Iago
Make: Cobra
Length: 5.8
Engine: Mariner 90 4-stroke
Join Date: Jul 2003
Posts: 42
A 5.5m plus 90hp would carry 4 divers plus gear without any difficulty (prop size may need to be thought out). I can't recommend going for a larger boat based on my own setup. Having said that, the 4-stroke is notably heavier but I could still see a two stroke 90 struggle with all the weight of the gear.
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Old 05 March 2004, 10:05   #5
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Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,850
Playsport, assuming that the Aluminium hull will be a similar weight to GRP then 90 hp will have no problem with a 5.5m hull and 4 divers. I used to have an Osprey Viper 5.25 with 75 merc. Four divers and 30 gals of petrol would give us a top speed of 24mph so 90 should give you that bit more. However if the cost of petrol isn't a problem why not put a V6 Merc on it ? great for the really flat calm days.

Not sure what you gain with the transom extention on a dive boat other than getting a smokey engine away from the divers, any chance you can test drive a boat from them before buying ?

If the console is forward it allows the divers plenty of room to kit up at the back and also means that they stand up when travelling to the dive site and can hold onto the back of the seat. If the console is at the back what are they going to hold onto or do they have to sit on the tubes neither very comfortable for long trips.

A wooden box up the front for weight belts will help to trim the boat and a small bottle rack to store the cylinders is popular in this country.

Pete
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