Starcraft Holiday senior project: Convert to twin Jetski drives

[drev500]

I was originally looking at using jet ski power for my current build but after some research found the following drawbacks:
1. Flat bottom boats don't offer a good surface for the intakes to get clean water, as soon as a flat bottom gets up on plane it gets out of the water and you're starving the jets for clean water.
2. Jets ski jets are pretty bad for efficiency, you have the poor fuel economy of a 2 stroke combined with the inefficiencies of a jet. Expect your flat bottom boat to be very expensive to feed on the water. You could probably feed a big block inboard for less $.
3. Jet ski motors are pretty notorious for their short lifespans, expect to rebuild one or both motors within short order if not before you put them into the boat. Jet skis get ridden hard and put away wet. Most manufacturers are pulling a lot of HP out of a small package to begin with.

Usually people take their jet ski out for a rip that lasts less than an hour and they don't really notice the poor efficiency of the drive setup because they don't really use them much. The motors last because they don't really get a lot of hours on them. A fishing/runabout boat generally gets used for hours at at time, you're talking about using two jet ski power trains so it will be like feeding two jet skis at all times, expect a day on the water to cost a few hundred dollars in fuel. That is comparable to running a big block attached to a berkley jet setup, except the big block will get you moving 20-30mph faster for the same fuel. Another limitation you will be stuck with is weight, most skis are considerably lighter than your boat is going to be, this means the impellers in your ski drives are probably not the right pitch and added stress on everything.

Jet drives generally deliver 2/3 of supplied power to the water. Meaning that a well setup prop will convert 33% more power to into thrust than an equally setup jet drive. So with a jet you're throwing away 33% of your potential speed and 33% of your fuel to get 66% of the potential performance of your motor.

As has been said it takes a specific hull design to successfully accommodate a jet ski heart transplant. Without significantly redesigning the hull in your boat you will probably find you have a boat that is pretty slow, terrible on fuel and expensive to maintain. You should really be grafting the bottom portion of the ski hulls into the bottom of your boat, just to feed the drives with water.

If the point of your project is to do something cool then I would build a custom inboard setup using whatever power head you want, coupled to whatever drive you want. If your project is to use a jet for the ability to run in sketchy or shallow water then I would hunt for an old donor boat with rotten floor and a running V8-berkley setup, this will likely be cheaper than the donor jet skis you are gonna buy. If you want to really do something cool that will run in shallow water with the older hull you have then l would build a custom inboard surface drive. I would power it with an older outboard power head that can turn high RPM and fab up the rest, or use an already marinized V6-V8. Locally I have see 2 running merc tower of power motors with pooched gear cases selling for a couple hundred bucks and complete running setups for a couple hundred more, buy the motors and sell the gear cases to pay for your customizing of the power head. Then all you gotta do is build your drive setup, no oil pan means the motors can practically lay on the floor of the boat with the crank shaft maybe a 4-5 inches from the water, couple it to a custom surface drive and you're laughing. Sounds like a lot of work but I bet its less to setup than the jets.

Supar,

The reason we chose jet ski was due to the relative size and ease of handling them. I have already purchased them (Yamaha Waveraider 1100).

I considered going with a Berk pump but with I decides it would be a lot of weight for this boat. With the engines being all steel you're looking at +600lbs plus the pump which is another 100 lbs. The jet ski motors are roughly 150 with pump so a total of 300 versus +700lbs.

I understand the efficiency issues with 2 strokes which I will have to deal with. Also, the old 1/3 loss of power really applies to outboard pumps. They use a centrifugal style pump which is much more power hungry than the axial pumps jet skis use.

This boat does have a V style hull but it is what Starcraft refers to as "gullwing" as the hull at the transom has a subtle curve to it.

These jet ski motors were used in a jet boat yamaha produced called the 210 Exciter. Those boats would do 50+. I understand this will be different and we will be doing studies to make sure we adapt these motors and pumps properly. I am already assuming new impellers will be needed but that will come after we test the boat's performance. For now, it can only be assumed.

 

[Suprathepeg]

Supar,

The reason we chose jet ski was due to the relative size and ease of handling them. I have already purchased them (Yamaha Waveraider 1100).

I considered going with a Berk pump but with I decides it would be a lot of weight for this boat. With the engines being all steel you're looking at +600lbs plus the pump which is another 100 lbs. The jet ski motors are roughly 150 with pump so a total of 300 versus +700lbs.

I understand the efficiency issues with 2 strokes which I will have to deal with. Also, the old 1/3 loss of power really applies to outboard pumps. They use a centrifugal style pump which is much more power hungry than the axial pumps jet skis use.

This boat does have a V style hull but it is what Starcraft refers to as "gullwing" as the hull at the transom has a subtle curve to it.

These jet ski motors were used in a jet boat yamaha produced called the 210 Exciter. Those boats would do 50+. I understand this will be different and we will be doing studies to make sure we adapt these motors and pumps properly. I am already assuming new impellers will be needed but that will come after we test the boat's performance. For now, it can only be assumed.

Yep I was looking at those power plants as well, they have a lot of punch for their small size.

Exciters are pretty peppy little machines for sure, however efficient they are not. My old man has a 135 merc behind a 17' V-hull similar to yours and it tops out at 55mph and it uses a LOT less fuel than an exciter.

That aside, if you cannot be dissuaded the first thing you need to do is take a lot of measurements of a twin engine exciter hull, confirm that the intakes your have from the jet skis match what they used. Also take a look at the hull profile and how it feeds towards the intakes. Jet hulls are very carefully designed to efficiently feed clean water to the intakes.

You are probably going to have to reprofile the hull and specifically the keel of the boat you have, subtle differences can have a huge effect on performance.

 

[drev500]

Yep I was looking at those power plants as well, they have a lot of punch for their small size.

Exciters are pretty peppy little machines for sure, however efficient they are not. My old man has a 135 merc behind a 17' V-hull similar to yours and it tops out at 55mph and it uses a LOT less fuel than an exciter.

That aside, if you cannot be dissuaded the first thing you need to do is take a lot of measurements of a twin engine exciter hull, confirm that the intakes your have from the jet skis match what they used. Also take a look at the hull profile and how it feeds towards the intakes. Jet hulls are very carefully designed to efficiently feed clean water to the intakes.

You are probably going to have to reprofile the hull and specifically the keel of the boat you have, subtle differences can have a huge effect on performance.

Sorry for my last post. Flagged with many grammatical errors, as what always happens when I type on my phone.

Its not that I am being stubborn but I already have the skis at this point. I unfortunately do not have the time or money to to go with a Berk pump and motor. The fuel consumption will be poor but I am ok with that. Maybe further on down the road it can be repowered with 4 strokes but at this time, the 2 strokes were the best option for money reasons. This project is completely funded by me and not having much income come in really limits what I can do.

I have studied twin jet drive boat hulls and plan to implement what those build had designed into this boat. With the jet pumps, you need to have smooth transitions into the intakes which will prevent cavitation. Strakes and such other hull features need end atleast 3 feet in front of the intake to allow for smooth water flow. Flat bottom boats do not allow for a viable solution for venting off the air that gets trapped under the boat when on plane. Air is a compressible and does not work well in a jet pump that relies on compressing water to produce thrust. The hull geometry of this boat will allow for air bleed and give the intakes a clear channel of water as we can place the intakes inbetween the strakes which will aid in water ingestion. There are different kinds of intake grates that help feed water into the pump. Also, in some applications people had used intake spoons. The spoons are placed in front of the intake and create a Coanda Effect with the water.

There are a lot of things I wish I could do but at the end of it, money rules and right now I am "ballin on a budget." Going to have to get used to the busch light for next few months...

We did a nice kick back today. On of my teammates used to co-op at Evercoat and they are hooking us up with 3 gallons of epoxy and some fiberglass mat...SWEET!

 

[Suprathepeg]

Generally the intakes are on the lowest point of the hull, I'm not sure how you plan to accomplish this with your convex hull shape. You need the intakes submerged but if you submerge too much of the hull you get a lot of drag. I wonder if you can get a drawing of the profile of the exciter?

 

[drev500]

The sides of the hull have a more pronounced curve than towards the keel. I was able to put a straight edge on the hull where we roughly think the pumps will be mounted and it lay flat. We are going to construct boxes that the pumps will be mounted into giving them a more modular fit. We are sketching some plans now of that, will post shortly. I am dreading buying all this aluminum.

 

[Suprathepeg]

The real trick is going to be rebuilding all the bulkheads and various braces that keep the hull shape what it is. Welding aluminum is difficult in the best circumstances and probably not a good plan with an old boat hull. Starting with a flat bottom hull and grafting the ski hull bottoms onto it might be cheaper and a lot easier in the long run, plus flat bottom hulls do a lot better in the water you want to run in.

Keep in mind that a boat depends on attaining a certain level of efficiency for it to work. Even though you're building off a deep Vee design that boat you have is a planing hull and not a displacement hull, so everything about that boat is build around the idea that when moving at anything above trolling/idle speed its going to be planing. This is the real trick you need to work around, you need to get the hull on plane AND still have clean water going to the jets or it will just hole shot, cavitate then dive and repeat... That will kill those ski motors REAL fast, every time it cavitates the motors are going to want to over rev and then when the hull gets clean water its going to be slamming the power train with resistance while the motor is revving at its outer limit. Its like taking your car redlining it and doing neutral drops over and over and over and over... motor and jet components will fail. If you can't get the boat up on plane properly the hull is going to be experiencing much more drag then designed while moving which means it will need a lot more power and torque to move it, those ski power trains are designed for a light hull that basically runs planing just high enough so the pumps won't cavitate, basically the only drag on a ski hull is the water intake. On a jet ski there is very little drag, the motors etc are all optimized to run like that, this is how they can pull more HP out of the power heads that they might with say an outboard using the same long block. Boat hulls like yours are why they make outboard jet drives, it allows them to run in sketchy shallow water but not have to worry about hull design.

My point is that you're going to need to keep those jets as close to center on the bottom of the hulls as you can squeeze them, look at the exciter I think they are about 18"-24" apart the cleanest water is going to be at the keel. The Exciter has a very shallow Vee hull design that is optimized to feed the jets with water, its also why they can't ever really get going as fast as the power on hand, with an outboard that hull would probably be capable of another 10-20mph using less fuel.

 

[Suprathepeg]

The sides of the hull have a more pronounced curve than towards the keel. I was able to put a straight edge on the hull where we roughly think the pumps will be mounted and it lay flat. We are going to construct boxes that the pumps will be mounted into giving them a more modular fit. We are sketching some plans now of that, will post shortly. I am dreading buying all this aluminum.

Why build boxes when you already have optimized jet ski hulls? 90% of the successful transplants use the bottom of the hulls, the motor mounts etc are all optimized for the power train, you would have to rebuild all of that exactly the same.

 

[drev500]

So how much Epoxy should I expect to use on the boat floor? We were able to get some 3 gallons and some mat. I plan to use exterior grade plywood. Should I add mat to the whole surface or just the seams? The sides and underside will get a coat of epoxy also. The finish is still up in the air but I want to get some floor down soon.

 

[drev500]

Grafting the jet ski hulls is something I do not want to do. I just don't see it looking good or coming out well. The pumps of the jetskis are able to come out as assemblies unlike Seadoos where they are molded into the hull. We can fabricate the boxes to allow the pumps to be more easily integrated into the hull. Trying to graft the fiberglass hulls into the aluminum hull sounds much easier than it would be in my opinion. We would have to cut large holes in the hull and make lots of reinforcements. I have thought about doing it that way but it looked a lot more challenging.

When we measured the motors and pumps yesterday, we are able to keep the engines at 10in. from the keel (that is on center of the motors). this also leaves a few inches in between each motor for working on them and other maintenance.

 

[Suprathepeg]

Epoxy doesn't adhere to aluminum very well. Usually on aluminum boats the sheets are covered with epoxy paint and installed individually. Then just caulk all the joints with marine grade caulking.

 

[Woodonglass]

I'm just wondering how all of this is going to be accomplished on a tight budget. Seems to me that this conversion is gunna cost more than you think and a lot more than a good used outboard motor and some plywood and foam to make the boat work as it was designed to work.

I'll continue to watch with GREAT interest. I sincerely hope your funds last as long as your motivation and desire. I LOVE to watch innovation as it unfolds. Extremely interesting to watch someone with motivation and desire pursue a goal. It's very intriguing!!!!:clap2::rockon:

 

[Willyclay]

Back in the day when Glasspar G-3's were not yet "classics", a bud and I had a plan to install a Berkley jet pump in one behind a BOP aluminum 215ci V8. It never happened. I hope yours does. Good luck!

 

[Suprathepeg]

Yea Drev500 don't take my comments as me telling you it can't be done. I think you've chosen a difficult path but, it can be done.

 

[Woodonglass]

Also you don't use mat with epoxy. Only cloth/fabric.

 

[drev500]

Yea Drev500 don't take my comments as me telling you it can't be done. I think you've chosen a difficult path but, it can be done.

Supra, I would never. I thoroughly enjoy your comments and concerns. These are the things we need to hear and be asked to make sure we make the correct choices. I understand the difficultly but we have a good group of young minds that are just as eager as I am in to get it done. I really appreciate you taking your time to voice your knowledge.

 

[drev500]

I'm just wondering how all of this is going to be accomplished on a tight budget. Seems to me that this conversion is gunna cost more than you think and a lot more than a good used outboard motor and some plywood and foam to make the boat work as it was designed to work.

I'll continue to watch with GREAT interest. I sincerely hope your funds last as long as your motivation and desire. I LOVE to watch innovation as it unfolds. Extremely interesting to watch someone with motivation and desire pursue a goal. It's very intriguing!!!!:clap2::rockon:

I understand the costs and am prepared to handle them. I'm glad Christmas is right around the corner as relatives will start asking what I would like. My list is endless hehe.

But Woononglass, should I just coat all the surfaces with the epoxy then paint? I don't know whether to use 1/2 or 3/4" exterior ply. The cost difference is not terrible but weight is an extra 20lbs for the 3/4" although I would really like the extra rigidness. Could the epoxy add that rigidness to the 1/2'?

 

[Suprathepeg]

What thickness are you replacing?

 

[drev500]

What thickness are you replacing?

So the only piece I was able to salvage was the bow section which is 1/2". The rest was a cluster and really soft. It was two pieces screwed together which looked like 1/2 to 3/4. I didn't get a close look I just started cutting and ripping apart.

 

[jbcurt00]

What thickness are you replacing?

On the deck (floor)?

1/2 is likely what was on the deck originally from SC, but if it was ever replaced, a PO may have used a different thickness.

AFAIK, no one has ever regretted using 3/4in instead or complained about the extra weight when finished. Sometimes the extra thickness effects alignment of side panels and consoles at put back, and that takes some time to resolve. But generally they rave about how solid the deck feels underfoot.

Some add extra bracing below decks to aid in supporting the 1/2in deck. Some are extremely diligent w rigid foam fitment below decks to aid in support of the 1/2.

If you can find it, 5/8 is a nice compromise between weight, additional cost and fitment/alignment.

The more you spend, typically, if prepped and applied correctly, the longer the sealing of the deck will last trouble free.

There are lots of ways to seal and finish the deck. Some seal and finish (including vinyl) the deck before installation into the boat. Makes the panels easily and individually removeable. Others dont want exposed fasteners or visible seams and install the deck after sealing and apply the final covering after. Vinyl has been done both ways successfully. Nautilux is used most often, there are other choices.

WOG has a boiled linseed oil old time sealer that works, as does straight spar, paint or epoxy. Epoxy MUST be covered w somethong else as most of it isnt UV stable. BLO, epoxy and spar can be painted over or covered w vinyl. Paint can be used under vinyl too, but I'd probably choose not to use paint if I was going to use vinyl on the deck. Grit can be added to all but BLO for traction.

Adding cloth to epoxy will stiffen 1/2 ply, esp if done to both sides. But epoxy and cloth will add expense and weight.

Got to weigh your cost constraints and weight concerns against boat building methods that match your skill level, budget and timeline.

Good luck and I look forward to seeing how you plan to craft the jets into the hull

 

[drev500]

On the deck (floor)?

1/2 is likely what was on the deck originally from SC, but if it was ever replaced, a PO may have used a different thickness.

AFAIK, no one has ever regretted using 3/4in instead or complained about the extra weight when finished. Sometimes the extra thickness effects alignment of side panels and consoles at put back, and that takes some time to resolve. But generally they rave about how solid the deck feels underfoot.

Some add extra bracing below decks to aid in supporting the 1/2in deck. Some are extremely diligent w rigid foam fitment below decks to aid in support of the 1/2.

If you can find it, 5/8 is a nice compromise between weight, additional cost and fitment/alignment.

The more you spend, typically, if prepped and applied correctly, the longer the sealing of the deck will last trouble free.

There are lots of ways to seal and finish the deck. Some seal and finish (including vinyl) the deck before installation into the boat. Makes the panels easily and individually removeable. Others dont want exposed fasteners or visible seams and install the deck after sealing and apply the final covering after. Vinyl has been done both ways successfully. Nautilux is used most often, there are other choices.

WOG has a boiled linseed oil old time sealer that works, as does straight spar, paint or epoxy. Epoxy MUST be covered w somethong else as most of it isnt UV stable. BLO, epoxy and spar can be painted over or covered w vinyl. Paint can be used under vinyl too, but I'd probably choose not to use paint if I was going to use vinyl on the deck. Grit can be added to all but BLO for traction.

Adding cloth to epoxy will stiffen 1/2 ply, esp if done to both sides. But epoxy and cloth will add expense and weight.

Got to weigh your cost constraints and weight concerns against boat building methods that match your skill level, budget and timeline.

Good luck and I look forward to seeing how you plan to craft the jets into the hull

Excellent info JB. Thank you sir (I hope I assume right as sir).

I like the idea of a removable deck. I was going to use a composite fuel tank I found locally that is 40 gallons for only 30 bucks, pretty darn good deal. Obviously used but I am going to look at it tomorrow so hopefully it is as nice as the seller says it is... I am going to place it in between the stringers but I was a little worried at its serviceability. Just piece of mind in case something came up with it, I would rather be able to remove the floor with easy instead of having to cut epoxy and drill rivets. Do you have a recommendation for fasteners that people normally use that sit relatively flush with the floor and will allow for easy removal?