Repowering AK work skiff with jet unit, advice needed!

[5akman]

I commercial fish in Alaska each summer in a shore based gill net fishery (set netting). My current work skiff is 23' x 8'6" built heavy in aluminum, powered by a '98 115hp Yama 2 stroke with 13.5 x 15 prop, top speed is around 25mph. We are in very silty glacial water, not much sand to speak of but lots of large rocks, as I found out last summer!

Use is motoring around shore tending our nets. I never motor more than about 1/4 - 1/2 mile from our launch point. If we get hit hard with fish, I could have 2000-4000lbs of salmon in the bottom of the boat. Current is about 6-7 knots as we tides going from -4 to +23 in a 6 hour period!

I'm trying to increase the responsibilities of my 3 teenage sons and would like one of them to take over the helm of the skiff. Since the silty water has about 4" of visibility, launching/retrieving/operating very close to shore with the large rocks is nerve wracking. A jet unit is the potential answer I believe.

I see my options as: a jet unit for my 115hp Yamaha or an inboard jet unit. With the 115 jet, I'm going to now be down to about 80hp which concerns me. Weight is a bit of a consideration with an inboard so I'm wondering if I go this direction, would a 4.3L GM V6 work with perhaps an old Berkeley jet? I don't need anything over about 20 mph but moving the heavy skiff and a load of fish against the current is a concern. Are there other aftermarket pumps to look at other than the Berkeley that might work better?

[baeckmo]

Just a quickie here, will come back with more later: yes the GM V6 may be fine, but certainly NOT the old Berkeley. It has the wrong characteristics for the job. There are probably lighter engines around that could do, but for commercial use available marine service is what counts.

[mohawk]

4.3 vortec would be perfect , but you should look for a flopro/hamilton/kodiac 2 or 3 stage jet unit. They are all the same thing and perfect for what you want.
A Hamilton 212 is also a good choice, but about twice the price.

My boat with 4 on board weighs about 1000KG and just jumps onto plane and gets up too 70+ kmph , with a 4.3 ( 200kg with ali exhaust,intake,waterpump,mounts ) and hamilton 2 stage.

[baeckmo]

Originally Posted by mohawk

4.3 vortec would be perfect , but you should look for a flopro/hamilton/kodiac 2 or 3 stage jet unit. They are all the same thing and perfect for what you want.
A Hamilton 212 is also a good choice, but about twice the price.

I disagree with your selection of jets completely there, those multistagers are high-pressure, low flow units not suitable for the job here. The impeller inlet area is what counts in 5akmans case. For good performance when loaded you need something like 240 mm inlet and with really good cavitation performance.

[mohawk]

The multi stage pumps are hi volume ,HI FLOW axel flow pumps, designed to lift, grip , and run with a grill half full of crap. Many guys agree the 3 stage pumps will out grip a 212 when the grills start getting blocked.

A 240 mm unit would be too big for a little 4.3.

You are right about the old Berkley ( A/T , dominator ) , It would work ok for what he wants too do, but a 2stage hami would drag it backwards.

[baeckmo]

With the loads we are talking about here, the two small units are out of question. You would have difficulties talking Hamilton people into using the 772 above 1700 kg's and the 213 (note the 212 is not for salt water operation) for anything over ~2500 kg. Attached please find a thrust diagram for four different units, all with a power input of 110 kW (150 hp), together with a rough first estimate of the resistance of 5akmans boat, according to the dimensions and weights quoted. To settle any disputes regarding what is high or low flows, I have listed rough figures for actual flow and pump head for the various types.

If you installed a 772 or 773 it would need ~190 hp to do the job and it would work in the cavitating zone so forget it, that's for running downhill in a light squirt hull. The 213 is better but still good not enough, remember that we are looking for a decent efficiency here, otherwise the fuel bills will eat the income.

It is extremely important that you understand the procedure of matching boat-engine-jet-cavitation. I get a fair share of customers needing help after having listened to "mohawk-ish" advice and ending up with jet boats that don't work.

So far the jet scene, for success with a jet boat you should be prepared to invest in at least 65 à 70 hp per tonne. If Iwere 5akman I would take a serious look into the possibilities of using a straight shaft with a conventional propeller "hidden" in a semitunnel above keel line. That would give better fuel economy, less weight and probably lower first cost. I can point you in the right direction if you need assistance.

[5akman]

thanks for the Thrust doc, interesting data. I'll give all this some thought and see what my options are.