What size outboard jet do I need?

[Vanislehunter]

This is my first jet boat. Have had a few suggestion on the size of motor I need and they were all quite different
I have a 16 foot welded aluminum tunnel hull.
It has 1/8” sides, 1/4” bottom. 62” wide at the bottom and 77” beam.
Super basic boat, just plywood floors and a bench seat at the bow.
I want the boat for hunting sheep, elk or moose on rivers with two guys and packs for 7-10 days.
Will be lighter weight backpacking gear not wall tents or anything like that.
Wanting to have a range of about 250km return
I am looking at buying a four stroke mercury outboard jet. What size motor will I need?

 

[LDUBS]

Max HP rating from the placard?

 

[Vanislehunter]

it doesn’t have one. Think only a few of these were made at a local boat builder.

 

[LDUBS]

Shucks, that would have made it easy. That hull is pretty robust. I would expect it to weigh quite a bit more than the typical 16' tin boat. I read the following in a google search. It is from Pennsylvania Fish & Boat Commission (whatever that is):

"Here is a safe guideline for selecting the right-sized engine: You need a ratio of 1 horsepower for every 25 pounds your craft will be pushing. Use the engine powerhead rating, not the jet rating, when using this calculation. For example, a 60/40 jet engine will effectively bring 1,500 pounds of gross weight on plane."

 

[FuzzyGrub]

Welcome to Tin Boats.

If those are the true side/bottom thicknesses, that is a very heavy boat! You will need the max size motor and you will still be leaving your friend behind with all the gear as you bring back the moose/elk. You will be hauling gas, be it 150 miles or 300 back and forth.

I'd calculate the weight of the boat and all your gear, people, etc, and look at the Outboard Jets website to see what range of powerhead you need: Boat Selection If it is even viable.

Given the transom, suspect that this is a prop tunnel boat, which are usually deeper/taller than typical jet tunnel. You might have to build the transom up and add jack plate.

 

[Vanislehunter]

Welcome to Tin Boats.

If those are the true side/bottom thicknesses, that is a very heavy boat! You will need the max size motor and you will still be leaving your friend behind with all the gear as you bring back the moose/elk. You will be hauling gas, be it 150 miles or 300 back and forth.

I'd calculate the weight of the boat and all your gear, people, etc, and look at the Outboard Jets website to see what range of powerhead you need: Boat Selection If it is even viable.

Given the transom, suspect that this is a prop tunnel boat, which are usually deeper/taller than typical jet tunnel. You might have to build the transom up and add jack plate.

Thanks, I’ll take a look at that.
Yes the bottom plate is 1/4. The tunnel is made with 3/16 and sides are 1/8.
This boat originally had a 25 inch transom but the previous owner had cut it down to 20 inch I’m assuming for a short shaft jet. I do not know the difference between a prop tunnel and a jet tunnel?
The transom looks very strong. I’m sure I could put a 90 on it if I wanted to.

 

[Billy H]

What a beast of a boat! Would love to know the actual weight of it.

 

[Zum]

I wouldnt want any smaller then a 40hp if propped...so 60 jet.
Bigger hp maybe better on fuel, depending on gear(weight)
I have know experience with jets, only what Ive read...keep this in mind.

 

[FuzzyGrub]

Thanks, I’ll take a look at that.
Yes the bottom plate is 1/4. The tunnel is made with 3/16 and sides are 1/8.
This boat originally had a 25 inch transom but the previous owner had cut it down to 20 inch I’m assuming for a short shaft jet. I do not know the difference between a prop tunnel and a jet tunnel?
The transom looks very strong. I’m sure I could put a 90 on it if I wanted to.

That is one heavy, duty, boat! On the plus side, it will be able to take a beating.

How far does the tunnel come up from the bottom, measured at the transom?

Might even want to go 105/80. In Yamaha, the weight is about the same as our 90/65. Not sure on Mercs, but though the 90 and 115 were about the same weight.

 

[Vanislehunter]

That is one heavy, duty, boat! On the plus side, it will be able to take a beating.

How far does the tunnel come up from the bottom, measured at the transom?

Might even want to go 105/80. In Yamaha, the weight is about the same as our 90/65. Not sure on Mercs, but though the 90 and 115 were about the same weight.

The tunnel is about 4.5 inches from the bottom at the very back. From the front to the back of the tunnel is about 48 inches and is 19 inches at the widest part at the back.
Yea the merc 115/80 is the same weight as the 90/65. I can get a really good deal on a 65 so I was hoping that was going to be enough power. Have had some people say it’s lots and others saying not enough…

 

[Vanislehunter]

.

 

[Still Afloat]

I

Welcome to Tin Boats.

If those are the true side/bottom thicknesses, that is a very heavy boat! You will need the max size motor and you will still be leaving your friend behind with all the gear as you bring back the moose/elk. You will be hauling gas, be it 150 miles or 300 back and forth.

I'd calculate the weight of the boat and all your gear, people, etc, and look at the Outboard Jets website to see what range of powerhead you need: Boat Selection If it is even viable.

Given the transom, suspect that this is a prop tunnel boat, which are usually deeper/taller than typical jet tunnel. You might have to build the transom up and add jack plate.

I was thinking the same thing. If you want a 250km range it would need a 60 gallon or so tank. Maybe more.

 

[FuzzyGrub]

The tunnel is about 4.5 inches from the bottom at the very back. From the front to the back of the tunnel is about 48 inches and is 19 inches at the widest part at the back.
Yea the merc 115/80 is the same weight as the 90/65. I can get a really good deal on a 65 so I was hoping that was going to be enough power. Have had some people say it’s lots and others saying not enough…

That sounds more like a prop tunnel, wider and deeper than typical jet tunnel. Don't know if there is much impact to performance. Maybe a benefit to have the motor higher such that the heal is not below the bottom?

If your river running doesn't require the shallowest possible, maybe a raised prop motor would be better? With cav plate at the top of the tunnel, or possibly a little higher, A prop would be much more efficient than the jet, power and gas usage wise. I have never been in or seen a prop tunnel boat, so would want to ask around on that to get more details.

I assume the boat builder is no longer in business to get info from. Given no boat specs, might have to take to a weigh station to try to get a real weight. If you do go "big" on the jet, I'd still add some good corner/transom bracing with the transom rebuild.