jet prop (swapping unit)

R

Ryan Tucker

New member
I am researching ways to get my boat into skinny water as cheap as possible. Someone suggested switching out my prop with a jet prop lower unit...

I know this requires raising your transom. How do you do that?

Also, I how do I go about knowing how my boat will run with a jet prop...I currently run a 2 stroke 70 Yamaha, I know I will loose 30% power, is my boat to heavy for a jet prop? I know weight is important.

I run a Center Console Polar Kraft
Length (ft) 17' 0"
Beam (in) 80"
Bottom Width (in) 54"
Side Depth 23"
Transom 20"
Aluminum Gauge .100"
Persons / Capacity 4 / 600 LBS
Dry Weight (lbs) 719 LBS
Weight Capacity 1160 LBS

Anyone know the pros/cons of swapping my prop for a jet? Or should I look into just buying a jet?
 
you can raise your transom with a fixed jack plate or you can have a welder build it up to the desired height using aluminum plate and tube. a good welder should be able to figure out the right materials and proper structural support to build it up. I had a smaller jet (20hp) and just had a 1/4 aluminum plate bent and braced that bolted onto the transom to raise mine.... for a heavy motor you will need more bracing however.

as for HP - that is a big boat and jets lack torque. When you start adding weight, they can really struggle. Get the highest HP you can afford and still get insurance coverage for! You'll probably want to be looking at a 115hp powerhead if you are used to a 70hp. That should come out to be about an 80hp at the pump.


Another option that is probably cheaper is to buy a jack plate and have a tunnel built into your hull... done properly it can get you in and out of some pretty skinny water.
 
Ryan,

i too, like many before me, looked into many ways "getting into the skinny water". my purpose was to get into (and out of) backwater tributaries at low tide. so i bought a 15ft tunnel hull jon boat. the tunnel hull effectively raised the motor about a whopping 3 or 4 inches higher than a normal short shaft. then i researched installing a jack-plate and low-water intake for my motor to get more clearance. what i learned, if i remember correctly, is that the jack-plate and low water intake are for larger boats and motors..and it was just too much money for the "advantage" that would be added. i could get into and out of the backwater by simply tilting the outboard motor..and poling when i have to.

the only thing i know about jet outboards is that they're LOUD. I assume that would apply to any jet prop.
 
What kind of "skinny" water are you trying to go in? If it is vegetation a jet will suck it up and plug up.
 
thats a good point... another thing they hate is ICE. not sure on your location but don't plan on breaking any ice over 1/2" thick... the broken chunks will also get stuck on the intake.
 

[font=Verdana,Arial,Helvetica] I am researching ways to get my boat into skinny water as cheap as possible. Someone suggested switching out my prop with a jet prop lower unit...

I know this requires raising your transom. How do you do that?

I bolted two 3/8" X 3.00" X 3.00" X 24.00" pieces of aluminum angle to the original transom knees effectively extending them up to the height I required. Then bolted a 2.00" thick transom board to the extended knees. The engine was then mounted to that. My transom was originally 15" and ended up at 25" for the jet set up.

Also, I how do I go about knowing how my boat will run with a jet prop...I currently run a 2 stroke 70 Yamaha, I know I will loose 30% power, is my boat to heavy for a jet prop? I know weight is important.

First question is engine weight and boats HP rating. An engine with a jet pump will weigh more than the same engine with a propped lower unit. My boat was rated for a 25hp. The jet pump I put on it had a 25hp power head that was rated 18 hp at the pump. The boat sat stern heavy until I moved both the battery and gas tank to under the center seat. Even then it tended to be stern heavy with driver only. It actually rode and performed better with a 5 gal pail filled with sand in the front as ballast.

I run a Center Console Polar Kraft
Length (ft) 17' 0"
Beam (in) 80"
Bottom Width (in) 54"
Side Depth 23"
Transom 20"
Aluminum Gauge .100"
Persons / Capacity 4 / 600 LBS
Dry Weight (lbs) 719 LBS
Weight Capacity 1160 LBS

Anyone know the pros/cons of swapping my prop for a jet? Or should I look into just buying a jet? [/font]

Ryan,

I owned and used a 14ft Alumi-craft Jon boat with a 25/18 HP jet out board for twelve years. It worked very well for what I bought it for and not so well for other pursuits. My main intent and use was to run shallow sand, muck and rock bottomed rivers for the purpose of fishing. It excelled at this. Good clean water and modest loads.

I did put a Avery blind on it two years after I bought the boat and started using it for duck hunting. It excelled in hunts that were river hunts, hauling one man, a dog and gear along quite well. A second hunter slowed it down noticeably but still worked ok.

Going to the marsh was a different matter. Jet pumps do not tolerate much vegetation.

Another downside no matter what the water type, is colder weather. Ice can be a problem, even more so if it is slush ice or snow slush that clogs the river. Slush will clog the intake even faster than broken sheet ice. Most of the time I was able to maneuver thru the sheet ice but slush will leave you stranded and you will be poling your way back out.


My typing really sucks and long replies are a chore for me. If you want to discuss this further I'd be more than happy to talk over the phone. My number is (319) 885-6687 and you can reach me most evenings after 6:00pm and before 9:00pm central time zone.


The best source for a jet pump would be direct IF they will deal with you. Their dealer network and your location to same is the determining factor. The only U.S. company making jets pumps is;

Specialty Mfg. Co.
2035 Edison Ave.
San Leandro, CA 94577

Phone; (510) 562-6049

 
but then qualify it by saying that I run a bigger pump than the guys that have said that "this" affects a Jet negatively....

first wt......the lower unit on the 150 powerhead JET that I have is LIGHTER by a good 20 lbs than the same lower unit with a prop......and thats apples to apples, (SS impeller/SS propeller), and is derived fro the mfg's spec sheet.....maybe that doesn't hold true in the smaller units....

second ICE......Jason Russel can be my witness on this one....last year while he was here we used my boat and jet to break through ice that was 3"+ thick for over a half a mile. We never had any problems with the jet being blocked by the ice.....even when we were stopped by the thickness of the ice, or when backing up to get a running start at it.....slush ice....again I'll refer to Jason as verification. On the last day he hunted with me, and after we broke through 1" ice, again with no issues, for a half mile, we ran through slush ice for tha good three miles at high speed, (40+ knots), with no problems.....at the end of the hunt we toured the bay and ran through slush again for a distance of over 5 miles.....I could feel subtle differences in the speed of the boat as the slush thickened, or thinned, but never lost more than a knot or two as we ran the bay. We also picked up a rig of goldeneyes in slush thinck enough to drag longlined rigs of 15 cork decoys with better than 10#'s of weight on each end with no problems with the jet...again this may be attributable to the difference in the hp rating of the unit.....don't know on that one as I've never owned a small jet.....

My own personal opinion is that I wouldn't own a jet that didn't have at least 80 hp rated at the nozzle.....anything smaller than that loses so much efficency under heavy loads that they become a detriment rather than an asset.....(if this is contrary to any person personal experience with their own boat/motor combination then I apologize in advance for slandering your combo....My opinion is based on my experience with smaller jets....perhaps the boat was too big or the load too heavy.....but then MAYBE NOT).....

My .02 and worth about the same as my belief that how you use a boat is an important part of how a boat will perform....

Steve
 
It's just that ALL the details weren't covered. I agree with you that HP may make all the difference in the world as to performance loss in varying conditions. Just to address the slush issue, I've gotten stranded twice in slush ice, conditions which as you stated, you were able to fly right thru.

Given the assumption that both pump units run the same clearances between impeller and cone, it would stand to reason that they have a similar spacing of the bars on the intake grate. Now it wouldn't be too hard to understand that 150 horses will pull the slush and water thru that grate, when 25 horses will only pull it up to the grate and not thru it.

I do think that smaller boat / motor combos regardless of prop type are more prone to power issues. I believe the reason for this is that it's just so easy to overload a small boat with out realizing it. A few extra pounds goes almost unnoticed in a larger boat but add those same pounds to a smaller rig and suddenly your underpowered and you start blaming the motor or hull shape or whatever.


Getting back to the jet HP size, you say 80 as a min. and for go anywhere, anytime, performance, I believe you are about right. However the smaller jets do have there place, but are more limited in their applications where they will excel.

[font=Verdana,Arial,Helvetica]"My .02 and worth about the same as my belief that how you use a boat is an important part of how a boat will perform.... " SO very very true, See, we even agree on this statement! hehe[/font]
 
in the success of breaking thick ice and running slush without getting clogged as well....my boat has 9 degrees of V so when its going slow, as in breaking ice the intake is deeper in the water than a lighter totally flat bottomed jon boat.....sames is true with the slush....I have enough depth to get clean water and speed before I get into the slush as the boat comes up on step, something that the lighter shallower drafted boats don't have.....

Works to a lesser extent on weeds...no one disagrees that a jet is worthless in thick vegetation if you are idling....but on some stuff, like topped out bonnets, eel grass, sago pondweed, stuff that doesn't form a solid mat that even an prop won't run through if I can get up to speed the I can run that stuff pretty successfully....likely a combination of the minimal V pushing the weeds to to the side, even slightly would be an advantage, and the increase suction of the bigger pump clearing it before it can build up.....I know that if something gets inside my bell that unless its a rock a stick or a piece of rope, its coming out, albeit in a much reduced size and consistancy....

I actually have more trouble with sticks, even little ones, and Coots then I do with weeds.....

Steve
 
Is there a difference between salt slush and freshwater slush? Doesn't salt water take a lot lower temp to solidify? Doest it pack slower?
 
sluch that you can shovel a trough in, that leaves a track of the boat in, is slush whetehr its salt or fresh....at least it seems that way to me.....

Steve
 
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