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Posts: 208
Location: Sun Prairie, WI | Thinking about upping the hp on my boat from a 75 to a 90. Same era motor. 2 stroke mariner. Getting a decent deal on the motor. What can I expect to see for performance gains. Should I start with the same prop as the 75? 17p aluminum. Also as long as the motor is off im thinkin about raising the motor 1 hole. Its about 1/2 inch above the bottom of the boat at the cavitaion plate. With the current setup im getting about 32mph at wot. still have alittle room to trim but starts to porpoise if i trim any higher. Boat is a 1988 yarcraft 1678 max hp is 90. any input would be great. thanks |
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Posts: 32934
Location: Rhinelander, Wisconsin | You should gain about 3 to 4 MPH. You may spin a 19 but the 17 should be good for general purpose. If the cavitation plate is above the running bottom already, I'd not raise the motor. |
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Posts: 395
Location: NW WI | sworrall - 7/19/2011 9:02 PM
You should gain about 3 to 4 MPH. You may spin a 19 but the 17 should be good for general purpose. If the cavitation plate is above the running bottom already, I'd not raise the motor.
X2 - holeshot should be a bit better also with the xtra HP and the 17 prop |
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Posts: 247
Location: Uxbridge Ontario | How heavy is the boat?
I have an 07 75hp Optimax and its stock prop is an aluminum 12-1/2" diameter 19P. It ran out to 5900rpm at about 40-41mph (light load). I now run a 13" diameter 19P SS prop with much better holeshot and the prop holds the water much better cornering running 5800rpm and about 39-40mph. I need to raise the motor one hole I think.
Who does your current engine do for rpm?
I went through a similar choice a while back. I almost switched from my 75 opti to a 90 opti to max the hp on my boat (06 Lund 1750 Outfitter SS) and the dealer I was going to buy the engine from talked me out of it saying the minimal gains will not really justify the additional expense. The only thing the switch would have gained was a newer warranty. |
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Posts: 208
Location: Sun Prairie, WI | wot rpm is right at 5000 where it should be. Boat weight is 750lbs acording to what I have found on the internet. Plus the trolling motor and 3 batteries and tackle. I usually run the boat arond 28 mph and around 4500rpm. Don't like running it wide open all the time. Think if I do the upgrade right it should cost me little to nothing to do. Thats really the only easy i am looking into it. |
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Posts: 797
Location: North Central IL USA | jjmuskie - 7/21/2011 3:28 PM
Think if I do the upgrade right it should cost me little to nothing to do. Thats really the only easy i am looking into it.
I do it for sure then! |
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Location: Hayward ,Wisconsin | Nope , not worth it . The little bit of difference it will make in speed will be lost in gas . IMO. Run it till you need a new one, had the 75 mariner on a 16' Basskoda , top end was 41 mph. Good motor .Don't fix it if it aint broke.
Edited by hog 7/21/2011 6:52 PM
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
Although many would disagree with me, I would say start by moving your current motor up one hole. Any time you can raise the motor a hole or two, you are gaining efficiency in running. Saves gas since there is less resistance in the water, and depending on your prop combination, you may notice very little loss in handling performance.
In almost all cases, the anti-ventilation plate is in it's best position when it is anywhere from 1" to 2" ABOVE the lowest point of the hull....reason: water immediately starts to fill the hole the boat made after it passes. If the anti-ventilation plate is in the water as you are at full speed, you are adding drag as well.
Any time you raise the motor, the trade off is a little less handling, which can be handled by a prop change. In most cases, going to a steel prop will increase bite on the water, tip cupping will further increase bite for lifting the bow and cornering, while cupping on the trailing edge will increase stern lift. That is why on bass boats you see very aggressive props with a bunch of cupping with a jack plate installed as well.
In your situation, going up from a 75 hp to a 90 hp will only give you about another 3 mph (roughly 10hp will be an increase of 1 - 2 mph) with a prop change and a height adjustment, you might be able to see up to 5 - 6 mph, but I would not expect much more. Your real gain will be in low end power. Hit the gas and you will feel it more in your seat.
I do have a question for you: What is the WOT range for that motor? If memory serves me correctly, that motor can turn up to 5500 RPM at it's maximum level. If that is the case, I would say you are OVER pitched by about 2 inches. One of the main indicators of being over pitched is lower RPM and the inability to trim up...there is so much downward push by the prop, the bow pops up quickly and the motor cannot keep it there. A lower pitch prop will lessen that effect, while getting you the extra RPM's needed to keep the bow up at higher trim levels.
So...my suggestion would be this: First, be sure your motor can reach it's maximum RPM's while lightly loaded (just you and your gear in the boat). If you cannot reach the max RPM's but are close, I would raise the motor one hole and re-test. If that does not get you there, go down in pitch. Get that max RPM.
I know when I was doing my testing, I found the best I could turn was a 17 pitch. As soon as I went to a 19 pitch, my motor could not turn it. I lost some 600 RPM's because it was just too much and was over what the motor could handle.
Steve
Steve
Edited by VMS 7/23/2011 8:34 AM
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Posts: 208
Location: Sun Prairie, WI | Well I will find out today or tomorrow if I get the new motor or not. Acording to the book I have the max rpm for the 75 is 5000 to 5500 and the 90 is 4700 to 5300. So Im thinking that the 90 with the same prop and go up one hole may get me right where I want to be.
I don't know how much life is left in the 75. Been acting kinda strange lately. Touchy to start one after fishing a spot and motor has sat for an hour or 2. also seems to fade out alittle on the top end. Just hopin the 90 is in better shape so I can get a few more years out of the boat before I get to buy a newer one. just did a compression check on the 75 and its around 115 on all 3 cylinders. any idea where the 90 should be for compression |
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Posts: 208
Location: Sun Prairie, WI | Ok, Well I picked up the boat and motor today for $600 Got it running and seems to run good. It has a 13.25x19p ss turbo prop. Do you think that is too much prop for the boat or should I just try it first. it was on a 17 foot deck boat so the weights should be close to the same. There is also a manual jack plate that sets the motor back about 5.5 inches. Is this something worth messing with? Would are the pros and cons of this setup? Thanks for everyones help. |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
I would try the prop that is on it first and see what it does. The jackplate will be a nice feature for you. I would set the motor so it is about 2-1/2 to 3 inches above the lowest point on the transom and see how it works. What the jackplate will do is two-fold....it will get your motor running in cleaner water (no bubbles from the hull to disrupt water flow into the prop) and it also moves the center of gravity more to the rear which makes trimming the bow easier. You will find you will not have to trim up as much to get the bow to lift. The only thing you might want to consider when using the jack plate is to get a water pressure gauge (they are around $35 or so) just to be sure you don't get too high and start losing water pressure. I believe staying above 10 psi is where you would want to be at to keep the engine safe from overheating. If anything it is a nice feature to have since it can also give you some insight into when a water impeller change could be needed.
The prop itself has a smaller diameter, but higher pitch, so you might be pleasantly surprised at how it might work. My gut instinct says it will be a touch high on the pitch, but that motor should have a little higher displacement than my yamaha 90 2 stroke, so it may work just fine. Nonetheless, if it doesn't work, you should be able to sell the prop for around $150 or so, recouping some of the money spent on the motor. And..if you decide not to go with the jack plate, you could get another $70 or so from that, if not more...
Good luck and definitely report back on how it is going.
Steve
Edited by VMS 7/23/2011 9:15 PM
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Posts: 208
Location: Sun Prairie, WI | Where would the water pressure gauge hook up to the motor? |
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Posts: 208
Location: Sun Prairie, WI | Ok well I think I found it. There is a plug in the water jacket just above the top spark plug. Is this where the water pressure gauge should be hooked up? |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
It might be. I know with mine, It was located on the side. If you have access to a service manual, that would help out. Might be able to find that online somewhere..
Steve |
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Posts: 208
Location: Sun Prairie, WI | Got both motors off tonight. Will hopefully have it all buttoned up tomorrow. As far as the water pressure gauge goes, what do I need? 15psi or 30psi |
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Posts: 208
Location: Sun Prairie, WI | Well took my first trip aroung the lake today. Seems to run good. Motor is about 1 inch above bottom of boat right now and Im getting 36mph and seem to be over revving just alittle. running at 5800rpm wide open. Dont know where to go from here. Raising motor raises rpm right? Bigger prop gonna take some of the hole shot away. Maybe just leave it and make sure Im not turning the motor that high. |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
If it has a max of 5500 RPMs and you are at 5800, you could go up 2 inches in pitch, and raise the motor one hole from where it is at, and you would be really close to that mark. Or....change to a steel prop of the same pitch and diameter and probably settle right in on the 5500 and gain some top end speed as well, along with maintaining a solid hole shot. If you go steel, there will be quite a bit less flex in the fins of the prop, which is a good thing for both bite on the water and forward thrust.
Steve |
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Posts: 208
Location: Sun Prairie, WI | wot rpm is actually pretty close to where it should be. maybe 5600. I was alittle nervous the first time running so I didn't look to closely at the tach. Second time on the water it is right where it should be maybe a tad over, but since I never run wide open all the time Im not to worried about it. Will raising the motor gain me rpm of lose rpm? I allready have a 19p stainless steel prop. Havent' played with the jack plate much since I don't have a water pressure gauge yet and don't wanna chance raising too much and losing water pressure. Had 2 people in the boat sunday and could tell the difference on the hole shot from the 75 to the 90. Much better. Now if I can just recover my expenses of the 90 by selling the 75 I will be happy. Thanks for everyones help!! |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
Raising the motor will gain RPM's...usually about 100 rpms per inch you move up.
Being as close as you are, If you wanted, you could still play with some different steel props and see what you find. you might be able to get it down to that 5500 with just you and your gear and still gain some top end without any real loss in holeshot..
That is the fun part of working to get your boat propped to your liking..you get to go out and play on the lake... 
Steve |
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Posts: 208
Location: Sun Prairie, WI | Ok, I decided to try my aluminum 17p prop today to see if the lowerunit clatter would go away. It did! Now the 17p alum runs better than the ss turbo. RPM's right at 5200 and getting 38mph. Think I will just run the alumn prop and sell the ss one. Any idea why the alumn would loose rpm and gain speed and the ss would over rev and loose speed? Turns out the ss turbo was repitched to a 18 and actually measures out closer to 17 acording to my local prop shop. |
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motor upgrade