It took me a while to spot this thread but here are some of my thoughts. As Doublespey mentioned I have a 12'2" Achilles with an inflatable keel and some serious fibreglas floorboards. I have also spend some time running around on the Morice/Bulkley so I have an idea of your needs.
First, you could probably get by without a jet - that is what Pete Pederson and a number of the "old boys" have done for years. Though they do have a "river runner" set-up which allows him to tilt the engine immediately via a handle attached to the top of the engine cover. It works while under power. As for where and what they are I am afraid I can't help you. Though I have seen others running rafts with props there without the special set-up.
A prop would aloow you to get away with a 15HP (but a 20 would be better). As for a jet, you could probably get by with the Merc Jet20. I used one up there on my boat and it was fine for just me, but I didn't think it had enough for 2 large guys. A jet28 (40HP at the head) is better IMHO. I think if you could find a 30HP prop and convert it it would be perfect.
As for max engine size, I have to admit that I have pushed the limit a bit as I have a Yamaha Jet40 on the back of my boat. Yes, a "tad" big, but the only insurance you have with a jet is power. One of the problems with rafts is their short length. This gives very little planing length, without the length (and weight) out in front the only thing to put the boat on plane is power. Another key issue in favour of power is the poor drag co-effient of the round rubber boat. The drag definitely hampers the speed and handling of rafts. If I were to put my power on a 16-17' hard boat I would leave a raft like mine out of sight in a matter of minutes.
Even with the negatives, the jet-raft is great. We took it into the Dean a couple of years ago - giant engine and all - and it was great to have. You can't do that with a hard boat (at least very easily)!
To summarize, go ahead with a raft, I would recommend a jet. Depending on how large you and your regular fishing partners are I think the Jet20 is not really enough for 2 guys - a 30HP prop converted with an aftermarket pump (as I don't think there is a factory one made - though I could be wrong) would be perfect for a 12' raft and 2 guys.
Tight lines - tyler.
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