bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

[Slimmdaddy]

Ok, anyone who followed my other posts are kinda up to par but for those who didn't I will quickly refresh.

Boat is a 1978 Cobra 17 ft

When I purchased this boat it had a 1978 Mercury 115 on it and would gps 47.8 mph with a quicksilver 21p. Since I have had race cars and hot rods since before I had a license I wanted some more power so I added a 1974 Mercury 150.

The engine had some gremlins but I got them straightened out and it is now screaming, I have a hard time believing it is only 35 more HP..

Ok, so here is where i am now. When I did the engine swap I added a 6" jack plate because I thought they were better for getting the engine in some cleaner water for more stability and mph....plus they look cool.

I am under the understanding that when you add a jack plate you raise the engine approximately 1/4"-3/8" for every inch of setback. So that is what I did. I started with the prop shaft 3 7/8" below the pad and the boat feels very slippery when I get on top of the water and start trimming it up. With the 115 on it there was no slippery feeling. The 115 had a 21p Quicksilver prop with kinda blunt round ears. That prop on the 150 felt like the engine just overpowered it when I hammered it hard...RPM would skyrocket and no more mph increase.

I now have a Trophy II 22p prop that looks much more aggressive but still doesn't feel like it is holding the water with any grip. The best I can describe it when you are driving and just spinning the tires and the car responds just like it is driving on something slippery.

I am getting to approximately 47 mph on my gps around 4800 rpm with very little trim, the more I trim the more rpm goes up but not mph. I was told by a local guy here that races outboards that to him it sounds like the engine is too high and he recommended that I go all the way down then work up in 1/4" increments.

Any suggestions or tips will be appreciated.

Thanks
Slimm

 

[wired247]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

Sounds like you are ventilating the prop. Start dropping the motor in 1/4" increments until you can hook up solid and run at top speed without sucking air into the prop. For a pad boat you are looking at 3-5" prop centerline below the pad on a 6" setback. You can go higher but unless you are running a surfacing prop like a chopper it wont do you much good. Watch your water pressure too. Getting i tthat high will starve the motor for water unless you have modded the water pickup. If you are at high prop centerline setting it wont do you much if any good to go beyond horizontal on your prop shaft. Remember trim is what you use to raise the prop when you arent using a jackplate. When you use a jackplate, trim becomes less effective and can cause problems.

Low 60's should be in reach

 

[Chris1956]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

Slim, When you raise the engine straight up, you lose some ability to lift the bow, using the trim feature. When the boat rides flatter in the water, you can lose speed. Look for a sweetspot between trim effectiveness and motor height.

Not sure about the squirrley issue. Sometimes when you hit high speeds, the hull becomes more unstable. I doubt that it your issue, as your hull is likely designed for high speed.

Enjoy that 1500...they were a bit high strung, but fun.. BTW, I plugged my top water intake hole and elongated the bottom one. Just to make sure she was cooling, as the A-V plate was a few inches above water.

 

[Slimmdaddy]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

Sounds like you are ventilating the prop. Start dropping the motor in 1/4" increments until you can hook up solid and run at top speed without sucking air into the prop. For a pad boat you are looking at 3-5" prop centerline below the pad on a 6" setback. You can go higher but unless you are running a surfacing prop like a chopper it wont do you much good. Watch your water pressure too. Getting i tthat high will starve the motor for water unless you have modded the water pickup. If you are at high prop centerline setting it wont do you much if any good to go beyond horizontal on your prop shaft. Remember trim is what you use to raise the prop when you arent using a jackplate. When you use a jackplate, trim becomes less effective and can cause problems.

Low 60's should be in reach

Thanks Wired, I think I will just lower the engine all the way down then make adjustments from there until I see improvement...then make smaller adjustments until I see improvement stop or get worse again.

Slimm

 

[Slimmdaddy]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

Slim, When you raise the engine straight up, you lose some ability to lift the bow, using the trim feature. When the boat rides flatter in the water, you can lose speed. Look for a sweetspot between trim effectiveness and motor height.

Not sure about the squirrley issue. Sometimes when you hit high speeds, the hull becomes more unstable. I doubt that it your issue, as your hull is likely designed for high speed.

Enjoy that 1500...they were a bit high strung, but fun.. BTW, I plugged my top water intake hole and elongated the bottom one. Just to make sure she was cooling, as the A-V plate was a few inches above water.

Thanks Chris, I sure love these old engines. These are the first old Mercury as I have had many old OMC Johnson and Evinrude and liked them well. This thing sure does like to go upstairs. I have a friend here that has a louvre press and I had him press some louvres into the engine cover to get some more air in there and I hope it helps. I will take some pics soon, just got it back today and left camera at home.

Slimm

 

[Slimmdaddy]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

Ok, I spent a couple hours on the lake tonight after work. I lowered the engine on the jackplate all the way down to start which put the center of the prop shaft 5 3/4" below the pad. I put the boat in the water and made some test passes, came back to the ramp and loaded on the trailer and raised the engine 1/4" at a time. I am now at 4 3/4" below pad and have made lots of progress I feel. I think I can raise another 1/4" or 1/2" but it was so windy outside and the water was so choppy later in the evening I just didnt feel like I would get consistent results.

I am now gaining mph very rapidly for small levels of trim adjustment and the boat feels like it is launching and pulling much better than before.

I will do some more adjusting tomorrow and hopefully have some more updates tomorrow night.

Thanks everyone for the help and advise.

Slimm

 

[wired247]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

So, how fast are you going?

 

[Slimmdaddy]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

So, how fast are you going?

Ok wired you asked the question I was hoping to avoid...lol

I hit 47 last night in some windy conditions. I am trying to learn the boat and its characteristics before just going out there and trying to show everyone I can run 60, or whatever. This is my first bass boat that is capable of higher speeds ( I drove my dads '79 Ranger with an Evinrude 70 for years but it was long and skinny tri-hull and topped out at about 34 on gps.

I kinda feel like people look at me like a scaredy cat but having driven drag cars since I was 14 and open wheel modified dirt cars I understand fully the effects of someone getting in over their head and not wanting to admit it...seen it too many times.

I am missing my right leg from the hip down since I was 7 years old and I do not know how to swim. The very last thing I would ever want to do is cause harm to someone on the water because I was trying to be the big bad fast boat guy and did something stupid. I think a little more seat time learning to cross wakes and how to properly drive the boat at speed when being passed by another faster boat or when meeting another boat head on and I will settle in fine..

Bet you didn't think the answer would be that long winded huh...lol


The boat with the 115 and very lightly loaded topped at 47.9 on gps...I have added 35 more hp, a bigger fuel tank and a jackplate and just trying to learn the boat again...I was surprised at how much different the boat handles and feels since the changes.

Slimm

 

[jimmbo]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

By Squirrly do you mean Chine walking? The hull starts to tip from side to side but also shifts abit left to right at the same time? Over trimming can induce this combined with the motor behind mounted too low for the speed.

If you were doing 48 with the 115, swapping to the 150 should put you at 55 with no other changes. raising the engine an inch and a half shoud be good for 1 to 2 more. Once you get over 50mph you will notice the boat will handle differently. As said before, raising the engine will result in less bow lift at any given trim seting. cafefull selection of props can do wonders, for handling as well as top speed.

That 150 likes to rev. It idles nice, but between 2000 and 4000 it isn't the smoothest engine out there, Big carbs and aggressive porting are the reason. Back in the early 70s Kiekhafer was against putting this engine into production because of its peaky torque curve, but when he left Mercury it was released. From 4000 to 6000 things happen fast. It smooths out and really hauls the mail. It puts out a lot of power from 100 cubic inches

 

[Slimmdaddy]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

By Squirrly do you mean Chine walking? The hull starts to tip from side to side but also shifts abit left to right at the same time? Over trimming can induce this combined with the motor behind mounted too low for the speed.

If you were doing 48 with the 115, swapping to the 150 should put you at 55 with no other changes. raising the engine an inch and a half shoud be good for 1 to 2 more. Once you get over 50mph you will notice the boat will handle differently. As said before, raising the engine will result in less bow lift at any given trim seting. cafefull selection of props can do wonders, for handling as well as top speed.

That 150 likes to rev. It idles nice, but between 2000 and 4000 it isn't the smoothest engine out there, Big carbs and aggressive porting are the reason. Back in the early 70s Kiekhafer was against putting this engine into production because of its peaky torque curve, but when he left Mercury it was released. From 4000 to 6000 things happen fast. It smooths out and really hauls the mail. It puts out a lot of power from 100 cubic inches

Hello Jimmbo and thanks for your info. When I say squirrly I mean the boat feels 'loose'. That is the best I can describe it from my racing days. The boat has not given any indication of chine walk yet. I have befriended an old Mercury technician in an adjoining town with 61 years Mercury outboard experience. He has a boat identical to mine that he uses for testing engines that people bring in for repair or rebuild and he has mounted engines up to 225hp on it and says it is a very fast hull that handles the power nicely, that is the extent of my knowledge on the hull and how it may handle at higher mph.

I have been busy this weekend and haven't taken the time to update as I should. I have the engine sitting at 4 1/4" below pad at this moment and I think I will lower it 1/4" down for a final setting of 4 1/2" below pad. Right now with myself and all my fishing gear and approximately 8 gallons of fuel, the boat launches like a rocket. as soon as it come up it revs to about 5000 rpm and then grabs the water and takes off so hard it will pull the hat off my head. I have friends who say their boat revs free like that but I don't think I want my 40 year old jewel to rev that hard free spinning the prop so I am going to try to tame it down a little. I don't run it as hard as possible every time I start it, usually just cruise and enjoy the beautiful sound around 4000 rpm..but if I am cruising at 4000 rpm and slam that hot foot to the floor and start trimming up, oh my goodness I cannot believe the response and power from this old beast.

A side note, I have set the timing at the factory recommended 23 degrees and am running either straight 100 octane fuel with a 40:1 oil ratio or I mix 110 octane fuel with 93 octane to keep the engine drinking a nice respectable fuel that I know will allow it to do what it does best without melting down.

I choose to run the 40:1 mix because I like the idea of that little extra oil in this old thing even though I keep hearing to run 50:1 or higher...that little extra oil couldn't be a bad thing I don't think.


Slimm

 

[emckelvy]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

You need to prop that motor to achieve at least 5500 rpm at WOT with a light load.

You are badly lugging that 1500 if it's only getting up to 4800-5000 rpm. Lugging the motor will result in burnt pistons and eventually damaged cylinders.

The 1500 will be quite happy to rev to 6000 or more but in deference to its age, 5500 is perhaps a bit less insane!

Perhaps it's more an issue with the motor setup, once you find the place where it's happy, you'll be able to do a WOT run and see what she does.

HTH.............ed

 

[Slimmdaddy]

Re: bass boat engine height with jackplate mercury 1500 very 'squirrely' at speed

You need to prop that motor to achieve at least 5500 rpm at WOT with a light load.

You are badly lugging that 1500 if it's only getting up to 4800-5000 rpm. Lugging the motor will result in burnt pistons and eventually damaged cylinders.

The 1500 will be quite happy to rev to 6000 or more but in deference to its age, 5500 is perhaps a bit less insane!

Perhaps it's more an issue with the motor setup, once you find the place where it's happy, you'll be able to do a WOT run and see what she does.

HTH.............ed

I wasn't inferring that it would only do 4000 rpm, rather that I just cruise at part throttle enjoying the water and the scenery...when I get the need for speed and pull her old ears back hitting 6000 or 6200 is no problem and it gets there quick fast and in a hurry.


Slimm