Jet Boat Build issues

[junkraft]

I'm new to this site and my questions should probably be directed to a specific forum....but I'll start here.
!992 Yamaha Waverunner III WRA650Q ........the engine and drive cut out to maintain alignment integrity and glassed into a 1962 12" MFG runabout. Starts and runs well on the "hose", but when on the water, won't come up on plane only obtaining 15 mph. Engine is bogged down like it is "over-propped". Possibly changing the impeller to something with less pitch and maybe trim tabs would help? Any positive feedback and suggestions from anyone experienced in this type of build would be appreciated.
Thanks

Buzz in Vermont

 

[oldjeep]

Re: Jet Boat Build issues

Interesting project. Looks to me like there would be really poor weight distribution with the seats in the back, which would make it tough to get on plane. Do you know where your center of balance is currently?

 

[moosehead]

Re: Jet Boat Build issues

Despite the chopped engineering that should be plenty of jet for that hull. Where do your intake grates sit within the hull? As OJ states above maybe it is too stern heavy to get proper intake. Perhaps you are not getting enough water pulled into the jets for full propulsion, either to far fore or hydrodynamics of hull causing cavitation? Are the impellar and wear rings in good shape? Is rear jet nozzle aligned properly to transom?

EDIT: Among other concerns, I would not want to be sitting just aft and at face level of an open engine and gas tank. Looks like an old top fuel dragster with where the cockpit was just aft of the powerplant, likely the origins of the saying "blows up in your face".

 

[NYBo]

Re: Jet Boat Build issues

Welcome to iboats!:welcome:

Very interesting project, and it looks like the 'glassing was done really well.

You say the motor seems to be bogging down, so the first step is to find out if the motor is healthy. Start with a compression test.

 

[ondarvr]

Re: Jet Boat Build issues

A jet will typically reach peak RPM's, or close to it, even if the boat isn't moving, so if it's bogging (RPM's dropping) look at the motor.

 

[smokeonthewater]

Re: Jet Boat Build issues

I happen to have a 10' jet n cat powered by a yamaha 701 and a yamaha WRIII with a 650

First question I have is how did the waverunner do before you cut it up... second, as above, compression test, third, what impeller do you currently have? if it's the stock aluminum one you NEED an aftermarket impeller as those stock yammis were terrible those years... Honestly you need one anyhow for the size of the boat... search ebay for a solas YB-SC-A and possibly you may even need to have it depitched a little for optimum performance....... what condition is your stator (pump) in? if it's badly chunked it will have an effect....

have you had the flywheel off? if you got the timing out of whack it can cause you issues

last thing I can think of right now is that both coil and cdi failures are common issued with these engines and I have had BOTH occur with the same symptoms... starts and idles fine and revs fine or at least ok out of the water but falls on it's face in the water

 

[smokeonthewater]

Re: Jet Boat Build issues

OH and FWIW a 701 or a 760 will both bolt right in and both will be a substantial HP gain which would be VERY welcome in that hull.... I hope otherwise but you MAY find out that you just don't have enough motor... a stock WRIII with that motor will do mid to high 30's with one person and I have to believe you have double the weight there.... if you can get on plane I'd think low 20's is about the best you can expect.

as mentioned above you likely have a weight distribution issue sitting in the back too... try moving the helm to the front if you can

 

[H20Rat]

Re: Jet Boat Build issues

As some one else noted, it *should* come up to full RPM regardless of the boat moving and if it's not reaching it there is a problem with the engine. If it is getting to full RPM then it's just too much weight (or not balanced properly) for that drive.

+1
And to go along with that, you don't change impellers in a jet like you do on a prop boat. If you have a jet powered boat, it doesn't matter if the boat weights 1000 pounds, or you swap it into a 3000# boat with all your buddies with. As far as the jet is concerned, its going to hit the same RPM. A prop powered boat would obviously be much different.

 

[crabby captain john]

Re: Jet Boat Build issues

Certainly a ride for a nonsmoker....

 

[junkraft]

Re: Jet Boat Build issues

First , thanks for all your replies. My original intent was to mimic a Jersey Speed Skiff. It all started with the a '92 Waverunner III , WRA650Q that a neighbor had...he hadn't run it for a few years and said the last time it was run it acted like it had carb issues. Anyhow, I made sure it had spark before I bought it for "cheap". Bought and installed a carb rebuild kit and a flushing hose adapter. Fired it up and it seemed to run well enough to continue. Then I found the '62 MFG and the project began.
The bottom of the waverunner was cut out to maintain the OEM position of the drive /engine and gas tank. I bedded and glassed this assy into an appropriate slot cut out of the MFG. Re-doing the transom as seen was required. All the controls had to be relocated as seen. First trip to the lake resulted in a poor run with engine bogging and an occasional misfire. I replaced the coil/wires/plug caps.....removed the choke butterfly and replaced with a pump primer unit. I also removed the oil pump, installed a cover plate and went to a gas/oil mix. 2nd trip to lake yielded similar performance results but it did start and run better. Home again.........compression test yielded 122 front and 145 rear. Yes, the top end needs to be redone and the reeds checked out. I have not inspected any of the drive components as it externally looked OK being a fresh water only runner.
What do I do next?..........I'd like +30 mph and there has been mention of bigger motor that will bolt right in. The impeller needs inspection and to replaced with a stainless unit . BUT with winter just around the corner and temps now in the 30's in the A.M., this project is now on temporary hold as other seasonal equipment needs there servicing time in the garage.
This project is now in winter storage. Questions are?...Is there hope for what I want for performance with the 650?. I hate to dump $ into this engine if its not what I need . A low budget projects has already turned into a money pit. Impeller replacement is a given. The boat float level with me in it and existing seating is OK. This is not a heavy hull. 10 gallon fuel tank up front assists with the trim. Possible trim tabs and a modified discharge shoot??? I know zero about jet skis but I am a veteran of outboard boat racing and 2 strokes from the 50-60'S.
Keep feeding me input as I will look for parts/etc. over the winter months.
Thanks to you all!

retired Buzz in VT

 

[Frank Acampora]

Re: Jet Boat Build issues



First, understand that if that engine delivered 50 HP new, the actual horsepower delivered to the water by the jet would be about 35-37. With that horsepower pushing the boat, 30 MPH is out of the question. You would be lucky to get 25.

My first impression when I saw the photos was that you were mimicking a Jersey Skiff. BUT, with only 35 HP and most of the weight in the back, I suspect that she will plow water and never plane unless you move your weight to the bow.
(I have a 10 foot tunnel hull with a 60 on it and she will plow all day, achieving only 10 MPH or so at full throttle, until I lean way over the steering wheel. Once on plane she goes well. Not the same animal, but the principle is the same.)

You also appear to have a very long exhaust pipe and even though it appears to be a large diameter, it may be strangling the engine at full throttle.

While you say you maintained engine/drive alignment, in the photos it appears that the engine is farther away from the jet pump that it would be stock. Personally, I would like to see a couple of good photos showing the hull bottom and also a couple showing a top view of the engine/pump

 

[junkraft]

Re: Jet Boat Build issues

Thanks Frank,
The exhaust is OEM stock with a piece of 3" PVC slipped over it to prevent any damage The engine, tank. exhaust, & drive are all in original position. Yes, the Speed Skiff image is what I was after. Moving the seating forward is not a favored option, however it may have to happen. A 760 motor may be necessary as it will apparently bolt up to the existing drive and install an aftermarket impellor. Where will it end? These "conversion installs" appear to be a black science with crazy people like myself attempting to have success. I still run an 10' ABR (APBA endorsed runabout) with a 1953 Merc KG7 (16HP) cranking out 46mph with my tired 65 year old 215 lb. body. I am still hoping for better on this project. Attached is the only other interior pic that I presently have available. Thanks for your reply.

Buzz in VT

 

[Scott Danforth]

Re: Jet Boat Build issues

unfortunately this will not run like your 10' ABR with KG7. small jet boats are notoriously sluggish unless you get over 100hp.

BTW, welcome aboard and please post pics of the Merc and ABR.

 

[smokeonthewater]

Re: Jet Boat Build issues

with a 760 you can likely get over 30 mph but not gonna happen w a 650

want to have some REAL fun there are kits to swap in a 900 or 1100 kaw or yammi tripple! That's 120 hp and AWAY you go

Using a fuel tank on the bow for ballast seems like a good idea at first until you consider that your boat will constantly become more and more stern heavy as you run and you have no control over it... put your fuel as near to the center of balance as possible and move cargo IE people to the front if you want it to run right... Some smart tabs could help but as long as you have all the seating in the rear of a boat not designed for it you will be trying to overcome an unbalanced hull IMHO

you CAN add a wedge in front of the nozzle but I don't think it will be nearly enough to balance it out

I suppose you could move the engine all the way to the bow b ut then you are working REALLY hard just to put seats against the transom

You won't get quite max rpm while plowing but it SHOULD get fairly close... you ABSOLUTELY DO need a different impeller for a heavy boat tho than a lighter ski and holeshot is where the biggest difference is

It sounds to me like you never did have the ski tested and running right before starting the project... If you are a good shopper you can pick up another ski with bigger engine for next to nothing over the winter

 

[smokeonthewater]

Re: Jet Boat Build issues

Thinking on this more.. enough HP will likely overcome the stern heavy holeshot issue but porpoising will likely become the next issue.... smart tabs may fix that issue tho.... I still think the boat will run better with seats moved forward but MIGHT still work ok w em in the back

 

[junkraft]

Re: Jet Boat Build issues

Sources for engine swap kits and suitable impellers would be appreciated. I have too much time and $ in installing the jet drive assy/motor mount in the MFG hull to change it over. A bolt in replacement such as the 760 with a new impeller sounds like the most practical approach to start with. The current 650 is in really nice condition and a hone/ring job would probably bring it up to snuff..........but apparently still not enough power. The search begins and the saga continues.
Thank you all for your replies.

Buzz in VT

 

[junkraft]

Re: Jet Boat Build issues

One more question (for now).......what year 760's will bolt up to the '92 650 motor mounts to keep the current shaft alignment?

Thanks

 

[smokeonthewater]

Re: Jet Boat Build issues

Do some searching on "wave blaster 1100"
The yamaha wave blasters use the same engines and mounts as your wr650
So swaps will be the same with exception of the exhaust which will be much easier in your boat

Here is one of the options
https://www.extreemthrottleinc.com/ProductDetails.asp?ProductCode=kawi-b-1-conversion

 

[smokeonthewater]

Re: Jet Boat Build issues

Part - ADA 1200 Conversion Kit

 

[smokeonthewater]

Re: Jet Boat Build issues

760's ran through the late 90's and AFAIK all of em should fit
I believe the only differences are in carbs and exhaust.

History of Yamaha Waverunners Production Models - K38 Water Safety