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Some normal people speeds:

1989 Voodoo with full interior (seats removed for racing), bottom still hooked, weighed approx. 730lbs ready to race at OPBRA Peterborough 2017 with the 2cyl 55hp motor..

-First got the boat in late 2016: 1997 Yamaha 70hp 20" shaft, stock large gearcase, on the transom, 21" and 23"p aluminum props: 49 to 50mph.
-First race TORC Gravenhust 2017: same motor as above, small gearcase 2.0:1 gears, Ron Hill 12x19 thru-hub semi-cleaver, 3" setback jack plate: 54.5mph.
-Later in 2017: Yamaha 55hp stock 20" shaft motor, same 2.0:1 gearcase as above, same jack plate, 12.25x19" OMC over-hub cleaver: 55.5mph.
-Last test run in 2017, cold day, same 55hp motor/setup/gears as above, testing Kawartha's work on the Ron Hill 12x19 thru-hub semi-cleaver: 56mph.
-Late 2018, different (older, 1987) Yamaha 70, long shaft, small gearcase 1.86:1 gears, 12.25x19" OMC over-hub cleaver: 62mph in ideal conditions, 61mph no problem. Pulls hard to 6500rpm. Fun as hell. ..all of the above was fun as hell.

Tom's 4 Stroke Merc 60 boat at Peterborough 2017 weighed the same as mine on that crane. He was noticeably pulling away from me in the straights. I'd assume he was at like 58 to 60 mph there.?

Early 2018, when the 2Cyl 55hp was converted to 15" short and put on the Delta, I only gained approx 0.5mph for 56.5mph at best. (2.0 gears, 19" over hub prop noted above). And that boat weighed 650lbs at OPBRA Peterborough 2018.

Matt, I can add some more information on the 55 Yamaha performance and offer a suggestion.
We ran a pure stock 55 Mariner with a 1:71 case and a John Beer reworked 10 3/4 x 18 chopper....55-56 mph
They ran a Tuff 16 with a 55 yamaha and a 1:84 case at 55-56 mph....I don't know the prop.
You got similar performance on your VooDoo and Delta.

SUGGESTION:
I think it is the combination of reed stops and jetting that is holding the motor back on the lighter hulls.
So try no reed stops with plastic reeds, go up 4 sizes in the main jet and up 2 degrees in timing.
The motor idle should jump to 1100 to 1300 rpm...if the changes are working
Find some small diameter props 11 to 11 1/2.....a 1:71 foot is best with the small diameter, lower pitch props
IF? it goes better keep trying larger jets.
THEN blueprint the motor.....that will get you another 300 to 400 rpm.
It should run low 60's on the oval.

Matt, messaged me about my suggestions and didn't think they would work on his motor.
I always feel that the engineer who designed the motor knows more about it than myself or anyone else.
I was able to communicate with one of the Yamaha engineers directly in the mid 90's and he gave me solid advise.
REED Stops:
I am referring to the factory reed stop.....remove it. Yamaha has closed up the stop, in order to use the same block for lower HP motors.The 60C and 70B are examples....different reed cage....different jets.
The 55 hp yamaha is not a de-tuned motor...but it is 2/3 of a 90 hp yamaha...and just blueprinting the 90 stock motor is seeing over 100 hp.
EDIT: I USE TWO STAGE BOYESEN REEDS.

TIMING:
I have found that on race application stock motors with a 5500 rpm HP rating that raising the timing 2 degrees helped on motors running approximately 7000 in the race application. On motors running over 7000 I found retarding the timing helped. I depends on the motor and even same model motors can be different.

LARGER main jet:
On a 70 hp 6H3 you can run a larger jet in a 6H3 carb. provided you have the 70CES head.
Matt informed me that his motor runs rich so it makes no sense to go larger in jet size. GO SMALLER in jet size. The 55 yamaha was designed as a commercial motor and used world wide. It was not jetted to run 7000 rpm. That is why the motor runs rich.

GEAR CASE RATIO and PROP SIZES:
I think it is important to relate back to what Yamaha suggested.
The 60 hp Mariner, the 60DES Yamaha and the 70CES Yamaha....ALL had the 1:71 gear case.
The 70CES Yamaha came available with two prop sizes...11 X 19 and 11 X 21.
https://cdn.nettivaraosa.com/live/26290 ... -large.jpg
Those are the props you see on most T850 and S850 boats.
Our stock 70CES runs an 18 pitch........on the lake we can run up to a 22 but it is not good for racing.

WHY does Matt's boat only run 1/2 mile faster when its 80 lbs lighter.
The 80 lbs difference has to make lighter boat QUICKER. So the lap time is faster with the lighter boat.
TOO ME, the boats are running the same top speed because the motor is flattening out.
The motor is running at its max so the weight is not making a difference. But, there is more there.
The 2 to 1 gear case could be part of the problem.....that ratio is going to help a heavier boat and allow you to run more diameter.

BOOTTOMLINE:
Matt is running REALLY WELL. He is trying to get to the next level. THAT is the toughest challenge....you will try more things to get that needed 3 to 4 mph than before.....normally it is props and set up that get you there....not powerhead.

Don, thanks for the suggestions.

Just want others to know that they should consider doing some analysis before trying things. Some suggestions can be coming from people with experience with other motor models/types, contexts, or mix of setups and levels of tune. Or maybe they are trying to slow you down, ..

Learn to read plugs and piston wash before re-jetting? So far my experience has been that these Mariner/Yamaha 55hp 760cc motors run rich with the factory stock configuration.

I'm quite scared of advancing timing. If that goes wrong the block will be destroyed. There are so many factors involved there. Fuel, jetting, compression, air temps, max rpms, exhaust tuning.. Factory timing for the 55 2cyl is 26*, but factory max RPM is like 5500. So if we are wishing to be turning 6800rpm, do you really want to just blindly try 28*?.. I'd be scared to do so. But I don't know enough about testing timing yet.. (side note: Yamaha 90hp stock timing is 22*, so if that's the baseline for the suggestion for the 2cyl, see how this can maybe not apply?)

Reed stops: The reed stops provide a surface that relieves the stress of the flex of the actual reed, no? So I'd be scared to just remove the stop completely. My first guess/idea would be to try and still use a stop to soften the reeds' stress where it is fastened to the block, but somehow allow the reed to open more than it does in stock form. Again, this is just from observations on how the Yamaha stuff is designed. May not apply to other designs.

Don, I will have other gear ratios ready for testing soon. Can I try your props?

Trying to keep this thread about speeds in general, not about the Yamaha.. Was hoping to tell a bit of my story of progress to show others that improvements are possible.. Hoping it would motivate some, like I have been, by seeing others' work and progress. I've always considered all suggestions but I've also learned that not all of them "just work" as advertised. I've been using all suggestions and internet info as a starting point for ideas and/or more research..

There is a difference in the way you prep and test a motor with a spec sheet and a motor that does not have a spec sheet. If the motor has a spec sheet those specs must be matched. A 55 yamaha is one of many that are non spec T motors, so there are wider tolerances and worse you really do need to research and try different things.

YOU CAN LOOK AT OTHER NON SPEC T MOTORS THE SAME WAY.

Here's what we know:
- basic model[s] have been produced since mid 70's up to early 2000's
- for the most part it is a commercial motor and will run on low grade fuel if necessary
- its tough and durable with a good reputation. very well built.
- its 55 hp at the prop
- as a commercial motor yamaha will lean toward the safe side in performance and focus more on durability.

We need to find out what we need to do to get this motor that normally runs 5000 to 5500 rpm to run up to 7000 rpm.
Its all about fuel to air mixture, porting and compression.
The crank, sleeves, rods and pistons are the same as the 90 hp Yamaha and the 90 will turn 7000.....so the twin should do the same.
The head is different but the compression can be obtained to meet T class rules.

Now here is where the yamaha engineer reed stop suggestion comes into play.
We don't know if the read stop is designed on this motor to reduce the horsepower to 55 hp or not.....we do know the motor runs rich which makes it more durable. In the 1990's it was Boyesen 2 stage plastic reeds and they do not require a reed stop. IF? the factory reed stop is choking the motor a bit like it does with the 60C yamaha, then just this change could allow the motor to breath better, not run as rich and turn more rpm.

Commercial models run retarded timing because many of their motors have to run on low grade fuel. You want this motor to run on 91 octane Shell. So you should be able to bump the timing because of the better fuel and the need for higher rpm.

SURE YOU CAN........I have everything from 17 to 25 pitch.

Just to add to my collection I got one of the 4 blade 12 1/2 x 18 Ron Hill props. I have spoken to Jamie at Kawartha Propeller and I plan to get the prop to him next Tuesday for blueprinting. Not sure what the pitch will end up as, but we will see. It could be a very good prop for 3 cylinder drag...I will decide once Jamie looks at it.
https://www.ebay.ca/itm/13-Spline-Signa ... kL&vxp=mtr

that's great Don, I'll take you up on that for sure.

Other racers had good success with these Ron Hill Jeff Brown cleavers so I got a Jeff Brown Cleaver from Ron Hill this past summer in a size I thought would be good for my current setup.

Out of the Box, the this prop matched my current top speed, but it didn't quite feel smooth running, so I knew there was more potential in it.

This particular one did need extensive blade work to equalize all 3 blades, and also a basic sharpening/balancing. Kawartha did this work and I gained 1 mph over my previous best. So after getting that prop back from Kawartha, I was at 57mph with the Delta / stock 2Cyl 55hp. This is how I ran for TORC Haliburton 2018. I think the prop can do even better with more advanced fine tuning and thinning/sharpening...

I have heard feedback from some others that Ron Hill Jeff Brown Cleavers were really good right out of the box, without extra prop shop work.

Well with that being said......I will hold off on the rework.....IF? you want to try it before mid May... its at the house in Toronto.

The one I bought from Ron hill works pretty good. Im over 70mph with an out of the box 12 1/2" × 24p cleaver. Ive been wanting to get it worked but there is a ton more engine setup to be done first

That is interesting. Most run 23 pitch. Last year some top T750 OMC guys were looking to try 24 pitch. What boat/motor combo is it on and what rpm are you getting.
thanks for the input
Don