Motor Height Setup

[wmk0002]

I have an Alumacraft 1648 NCS flatbottom with a 15" transom that I recently mounted a 1988 Yamaha 30hp longshaft on. In the fall I installed a Panther 55 trim unit which has about 7" of setback I believe. I used 2 pieces of 3" x 4" x 0.250" 6061 angle to construct a riser/jackplate that would allow me to use a longshaft like this motor. I initially installed the motor in the top hole of the motors transom bracket which put the cav plate virtually even with the bottom of the hull. I think it reality it was still about 1/16" too low though. I lake tested it and it performed well - I was at about 6000 rpms and 30 mph with a Powertech SRA3 10 pitch. It was obvious by looking though that it needed to come up a decent bit more. I since moved it up one hole which puts it pretty spot on to 1" above the bottom now. I have one more hole on the motor I can raise it plus I can drill out and shift the angle brackets up 1-1.5"

I plan to tweak it but I had a couple of questions. Mainly just wondering if anyone had any idea what the final dialed in height may end up being so I can make mare aggressive changes initially? I also had some crazy tiller torque and really hope that as I go up that will decrease some. First run i had the trim tab/anode in the default position aligned with the skeg. I since have turned it to the right maybe 30 degrees to see how that helps. I also had a question about my prop. 10" just seems like a really low pitch for this motor no matter the hull so I bought a used SRA3 in a 13 pitch to also try. Comparing the props, the 10 pitch is in a lot better shape but it also has a ton more cup despite being the same model. When using a prop calculator how should I factor in the cup to the pitch input? When I put in 6k rpms, 10" pitch, 1.85:1 gearing, and 30 mph it says I have 2% slip which is obviously inaccurate. Should I bump the pitch up to an 11 or even a 12? All of that aside, if I have a worked prop does that mean that specific prop can be run even higher than a factory SRA3 like the 13 I bought? Right now I just want to get the motor height semi dialed in before I play with props too much and try to dial in my rpms.

 

[lckstckn2smknbrls]

The formula I went with when I mounted my mini jacker is for every inch of set back you can raise the motor a quarter inch. IIRC the mini jacker has 4 inches of set back so I was able to raise the motor one inch above the bottom of the hull. With further testing I ended up with my motor about 2 inches above the bottom of the hull. I can't help you with prop choice as I'm running the stock prop.

 

[wmk0002]

The formula I went with when I mounted my mini jacker is for every inch of set back you can raise the motor a quarter inch. IIRC the mini jacker has 4 inches of set back so I was able to raise the motor one inch above the bottom of the hull. With further testing I ended up with my motor about 2 inches above the bottom of the hull. I can't help you with prop choice as I'm running the stock prop.

Thanks. What kind of hull was this on?

 

[lckstckn2smknbrls]

The formula I went with when I mounted my mini jacker is for every inch of set back you can raise the motor a quarter inch. IIRC the mini jacker has 4 inches of set back so I was able to raise the motor one inch above the bottom of the hull. With further testing I ended up with my motor about 2 inches above the bottom of the hull. I can't help you with prop choice as I'm running the stock prop.

Thanks. What kind of hull was this on?

It's a 1971 Wards sea king 1436 jon running a short shaft 1983 Yamaha built 25hp Mariner. The motor is a longer short shaft about 17".

 

[wmk0002]

The formula I went with when I mounted my mini jacker is for every inch of set back you can raise the motor a quarter inch. IIRC the mini jacker has 4 inches of set back so I was able to raise the motor one inch above the bottom of the hull. With further testing I ended up with my motor about 2 inches above the bottom of the hull. I can't help you with prop choice as I'm running the stock prop.

Thanks. What kind of hull was this on?

It's a 1971 Wards sea king 1436 jon running a short shaft 1983 Yamaha built 25hp Mariner. The motor is a longer short shaft about 17".

Appreciate it! Bet that boat scoots!

 

[lckstckn2smknbrls]

It was fast no gps so I don't know it actual speed.
It was my duck hunting boat.

 

[nccatfisher]

Angles are deceiving but if you actually do have a true 7" of setback you should be able to be running that motor higher than it appears it is in that pic. I have 5" and am running mine considerably higher than than on a similar setup.

 

[wmk0002]

Angles are deceiving but if you actually do have a true 7" of setback you should be able to be running that motor higher than it appears it is in that pic. I have 5" and am running mine considerably higher than than on a similar setup.

I just checked the online spec and it lists 7" of setback. I have the camera positioned even with the bottom of the hull, which I set level prior. I forgot to verify if the cav plate was also level but it is pretty close.

 

[nccatfisher]

Angles are deceiving but if you actually do have a true 7" of setback you should be able to be running that motor higher than it appears it is in that pic. I have 5" and am running mine considerably higher than than on a similar setup.

I just checked the online spec and it lists 7" of setback. I have the camera positioned even with the bottom of the hull, which I set level prior. I forgot to verify if the cav plate was also level but it is pretty close.

With 7" of setback you should be able to run your cavitation plate over 3" higher than the bottom of the boat. Mine is over two I believe it is about 2.5"trimmed correctly. You will just have to keep raising it and see how it performs without porpoising, blowing out or starving the water pump.

 

[RaisedByWolves]

Angles are deceiving but if you actually do have a true 7" of setback you should be able to be running that motor higher than it appears it is in that pic. I have 5" and am running mine considerably higher than than on a similar setup.

I just checked the online spec and it lists 7" of setback. I have the camera positioned even with the bottom of the hull, which I set level prior. I forgot to verify if the cav plate was also level but it is pretty close.

With 7" of setback you should be able to run your cavitation plate over 3" higher than the bottom of the boat. Mine is over two I believe it is about 2.5"trimmed correctly. You will just have to keep raising it and see how it performs without porpoising, blowing out or starving the water pump.

Add about 5' of hose to your tell tale when doing this and run it up alongside the hull to where you can easily see it to prevent starving it of water.

 

[maintenanceguy]

Look up the specs on your motor and find out what the gear ratio is. With that, the prop pitch, RPM, and speed, there are lots of on line calculators that will give you prop slip. That will tell you if you're already in the ball park. With a 10 pitch prop, 30mph might be all your motor can give you. I ran a quick guess using a gear ratio of 2:1 - pretty common. I got a slip of negative 6 - which is better than perfect. So, your gear ration obviously isn't 2:1 but I think you're probably already where it needs to be. If you need more speed, you might need a different prop.

 

[wmk0002]

Add about 5' of hose to your tell tale when doing this and run it up alongside the hull to where you can easily see it to prevent starving it of water.

That's a good idea. I'd really like to add a water pressure gauge somehow too but I'll save that for down the road. Instead of having a traditional analog water fed one, it would be cool to have a mechanical sensor on the powerhead that was fed water and then sent a signal to light that could be mounted to the tiller that would flash when pressure lowered to a certain point similar to an overheat light.

 

[wmk0002]

Look up the specs on your motor and find out what the gear ration is. With that, the prop pitch, RPM, and speed, there are lots of on line calculators that will give you prop slip. That will tell you if you're already in the ball park. With a 10 pitch prop, 30mph might be all your motor can give you. I ran a quick guess using a gear ration of 2:1 - pretty common. I got a slip of negative 6 - which is better than perfect. So, your gear ration obviously isn't 2:1 but I think you're probably already where it needs to be. If you need more speed, you might need a different prop.

Gear ratio is 1.85. This 10 also appears to have noticeable cupping added to it. I think that in theory adds some pitch to it but maybe some prop pros can share some info on that.

In reality 30 mph is plenty fast, I just want to get it set up best I can so that I can determine if my motor is performing tip top. It has 40 carbs and Boyesen reeds but I'm not sure if it has a larger intake to match the carbs or if the reed cages are the larger ones. I have a set of factory carbs I will eventually try once I dial in the height so I can compare. If performance is similar I will just revert back to the 30 carbs so I can prolong the motor's life and maybe save on some fuel. Back to speeds, I'd just like to be able to get it to run mid 30's with just me and a light load so that I can carry another person or two plus more gear and still be at 30 or a little more. I've found the speed I enjoy cruising at the most in my boat is about 24 which prob puts me in a good medium rpm range but gets me where I need to go in a decent amount of time.

 

[nccatfisher]

Add about 5' of hose to your tell tale when doing this and run it up alongside the hull to where you can easily see it to prevent starving it of water.

That's a good idea. I'd really like to add a water pressure gauge somehow too but I'll save that for down the road. Instead of having a traditional analog water fed one, it would be cool to have a mechanical sensor on the powerhead that was fed water and then sent a signal to light that could be mounted to the tiller that would flash when pressure lowered to a certain point similar to an overheat light.

My Yamaha has one, but it is actually almost indiscernible in the daylight and if you think about it you aren't looking back near as much as you are looking forward. A LOUD alarm like the OMC low oil alarms to me would actually be better but that is just me.

 

[RaisedByWolves]

Those are both good ideas, but the problem is, believe it or not, these motors require 0 pressure to stay cool.


Its more about flow than anything so an overtemp warning is the best youll get.

 

[nccatfisher]

Those are both good ideas, but the problem is, believe it or not, these motors require 0 pressure to stay cool.


Its more about flow than anything so an overtemp warning is the best youll get.

Well it really isn't pressure in anything, pressure is developed in most situations in a closed system and also to lower the boiling point and even in a pressure system you have to have flow. Even in some systems that actually are pressurized with no water pump they rely on thermal conduction for flow. Some Kubota diesels were like that. But you are correct it is all about flow.

 

[lckstckn2smknbrls]

See if you can install a cylinder head temp gauge.

 

[wmk0002]

See if you can install a cylinder head temp gauge.

Can you get a sender that goes in the place of the factory thermo switch? One with varying resistance to work a gauge...but still triggers the warning buzzer?