1984 party kraft 20’ pontoon hp rating?

Smack3

Recruit
Joined
Sep 16, 2019
Messages
2
I bought an old 20’ party kraft that needed a complete redecking and some crossmember replacement. I changed the 16’ deck and extended to completely cover the 20’ (20” diameter) tubes and added some fabricated M-brackets for extra cross members. Replaced 4 -14ga crossmembers with 7 -12 ga which is substantially more rigid. The stern/transom mount use to connect to only 2 crossmembers but now there are 3 so it should be considerably stronger. I cannot find any information about the original hp rating on my boat, but I wanted to get some idea of the factors in considering upgrading my 40hp mariner 2 stroke with a larger 4 stroke. I think my tubes are smaller than current models and the existing M-brackets are only 4” wide on 24” centers whereas newer boats have an M-bracket that extends the length of the tubes. I still have not layed out the furniture yet as I plan to reupholster and possibly fabricate some fancier recliners and corners over the winter.

some other factors I would like advise on...
since I extended the motor back behind the tubes instead of between (approx 2’), how much will that impact the weight distribution? I was thinking of mounting battery under the steering console and possibly moving a little forward to compensate the weight.

so in summary
- any ideas on picking a max hp?
- thoughts on furniture weight distribution on larger deck?

my existing furniture was/is an injection molded plastic base, but I cannot find matching height corners or recliners so I may need to fabricate bases out of aluminum tubing or something? I don’t want to build out of plywood as I think that would be too heavy and will tend to absorb water. Any ideas where to find light base furniture that I can cover myself?

this is my first post on this site so hopefully you all will be kind with replies as I may be asking redundant questions, but I could not find any existing posts with similar topic.
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
The big thing to consider, especially when working with 20" tubes, is weight. This amplified by the fact you've moved the transom back. Transom support is important, but on a boat this size, it's going to be about weight. Check out the difference in weight among the 2 cycle twins, then triples, and the small 4 cylinders (forget about anything bigger than a small 4 banger, something like that WAY too heavy!).

With a clear understanding of the weight categories, I think you'll find selecting an engine much easier. Myself, I wouldn't consider a 4 cylinder. It would need to be lighter. How much lighter is going to depend on your floor plan, and how the boat will be used.
 

wahlejim

Master Chief Petty Officer
Joined
Jul 23, 2015
Messages
884
When doing the research on this, disregard the make and model and year. Focus instead on length and diameter of the logs. There is a lot of information out there on pontoon log buoyancy dynamics, not so much on the make and model of what your pontoon used to be. This is a custom build and should be approached that way.
 
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