Motor height....need some expert advice

[spego]

Ok here is my deal.<br /><br />18' Aluminum Spectrum Bass Boat<br />88 DT175 Suzuki OB 25" shaft<br />14.5 x 24 Suzuki Prop<br />Fixed Hi-Jacker Jacking Plate<br /><br />I have raised the motor all the way up on the boat. The reason is the motor is too big (25") for the boat. I bought the boat with the motor on it and the previous owner had a Hi-Jacker 10" setback and 4" vertical.....which was not enough lift. I took this off and am in the middle of selling it....and put on the fixed plate to get more lift...about 5". So now the cavitation plate is just about even with the bottom of the boat. <br /><br />Well with all that said....it is only pushing 53-54 mph WOT at 3800 rpm. It did at one time chine walk all over the place at about 50 until I took off the old plate and raised it further....that got rid of that. But I still am not getting the speed out of it. I really like the motor....it runs great and has TONS of power but I just cannot get the speed. Other than getting a different motor...anyone have any thoughts? I am considering looking for a way to bring the motor up more but the current setup is maxed.<br /><br />Here is a pic of the old setup....since I installed the new plate and raised it about 1.5-2 inches.<br /><br />

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[Winger Ed.]

Re: Motor height....need some expert advice

I think that's a off shore style engine. The standard fresh water engines use a 20 inch shaft.<br /><br />That would explain why you have to raise up it so high. And there is plenty go too, like getting the top of the prop arc about even with your arrow. The setback of a jackplate often lets you bring the engine up another inch or so from where it would hang on the transom.<br /><br />That 25" midsection sort of puts you in a pickle as far as getting the extra height you're after.<br /><br />About all you can do speed wise is drop the pitch of the prop, you may get a little more speed in exchange for higher rpms.<br /><br />The lower unit setting down that deep is a huge drag at speed. with that much of it in the water, you may just run out of horsepower at about 60mph no matter what prop you use.<br /><br /><br />The long and short of it is that you've just got the wrong type engine for the boat. You might look into selling it and getting a similar horsepower one on a 20" shaft, or check how much a 20" mid-case, shift link, water pipe, & shorter drive shaft costs.<br /><br />Another downside of the long shaft is that by mounting it up so high, the engine has a tremendous amount of leverage on the transom as it 'yangs' back (both in the water and while trailiering it)and forth putting much more stress on it than it was probably built to take.

 

[spego]

Re: Motor height....need some expert advice

Thanks for the advise on this! What I ended up doing is getting some assistance from a friend in a metal fab shop and he built a plate that will work. 1/2" aluminum and steel channel on the inside to secure it. Sucker is on there and not going anywhere for sure. Reinforced the inside of the transom too.<br /><br />So Friday I will get it out and let you know what the results will be. I enclosed some pics for you guys to look at. I will probably need prop advise after my run.<br /><br />I have to admit it does look a little weird with it up that high but oh well.<br /><br />

 

[Winger Ed.]

Re: Motor height....need some expert advice

I'm proud of ya. That's a great solution.<br /><br />You could tweek and tune for the next several months to get another couple mph out of it before you reached the point of over revving your engine.<br /><br />But where its at should do real well. The lower unit puts more drag on the rig than anything, so you want to get as much of it out of the water as possible and not have the prop cavitate.<br /><br />The way it is now, If it doesn't take off and accelerate like a car, just punch it......<br /><br /><br />I found that out with my hydro. If you try to just ease off and pick up speed like you were driving a car on a dirt road,,,, the prop ventilates with the exhaust, spins in its own air pocket,,,,, and won't ever get a bite.<br /><br />I found---- like I re-invented the wheel--- just punch it. Let it tach up to about 6000, for a few seconds, let off the gas enough to hold it there,,,,, then the prop will then bite, the tach will drop, give it more gas,,,,and you'll blast off like a dragster.<br /><br />On the surface, I'd think you get to about 65-odd mph with that set up. With a big enough pitch of prop that the eng. won't turn up over about 6000 on flat water. <br /><br />But, with enough up trim on the eng. to go that fast, you might not have enough hull in the water to steer unless you put a straight line mounted rudder or skeg permantly on the hull(facing straight ahead) about 2 feet in front of the engine along the keel. An extreme version of this is the turn fins you see on race boats and hydros either mounted on the keel of on one of the sponsons.<br /><br />At my photo site, you can see what I have on POCO LOCO in the photo where I had it hanging from the ceiling while I painted the bottom. My turn fin is bigger than what you'll need, but it lets the boat handle like a choo-choo train on RR tracks while going at speed. It'll turn as tight as you can hang on, and hold the steering wheel for without getting pitched out.<br /><br /><br />A piece of 1/4 Aluminum plate about 6-9 inches long welded, or bolted to a pair of 1 1/2 inch angle stock, bolted through your keel and hanging down about 3 inches should do well and let you steer enough to be safe at speed.

 

[BillP]

Re: Motor height....need some expert advice

I see rivets...they are a problem for high speed. Exposed rivets will never match speed with a flush bottom. I have a cousin who took a 14' skiff with 30hp and made all the bottom rivets flush. He said the speed increase was dramatic.

 

[spego]

Re: Motor height....need some expert advice

Yeah it has rivots on there. I will have to let them go....thanks for the input on that....never knew that. I am just trying to get the motor to operate in the 5600 rpm range it should for now. No speed records being broken today! lol Just looking to get the motor running in that range and maybe push it over 60 if possible (it should). Well when I test it I will let you all know. Thanks again.

 

[KCLOST]

Re: Motor height....need some expert advice

Are you trimming out to max?<br /><br />That can make a hugh rpm difference compared to being in the full down position...

 

[spego]

Re: Motor height....need some expert advice

Yeah I was trimmed out max before I made the adjustments to raise the motor. Maxed right out. Still only about 53 mph and 3800rpm. Like I said I will test it actually tonight and see what the difference is after the height adjustment. Let you know then.

 

[spego]

Re: Motor height....need some expert advice

Ok just got home and wanted to see what you guys thought about this. I ran it tonight and here are the results.<br /><br />4300 rpm 54 mph<br /><br />I actually thought it would improve it more than that but I guess I will live with that. I actually can trim it back till I get the 54 mph out of it....then I can trim it more and the prop starts to spray up and back and the mph go down slightly to about 53.6. That is right about where I got the 4300 rpm. The cavitation plate and hydrofoil are not in the water when running at 40 mph. When you trim it out all the way the water appears to be about a 1/2" to 1" under the plate. So from what I know it is set in the right spot. Basically there is no adjustments at this point that I am going to push dramatic amounts of mph out of it. So I will be satisfied with what I have but I need to know this now.<br /><br />What prop size do I need with the current setup to get the motor operating at the right range??<br /><br />The recommended operating range per the manual is 5000-5600 rpm. <br /><br />The current prop is a Suzuki 14.25 x 24 (3 blade). <br /><br />currently 4300rpm (max trimmed all the way out) @ 54mph<br /><br />Let me know what you think about prop size?<br /><br /><br />P.S. - I do also have a turbo 2+2 (4 blade) 13 3/4 x 26 which lowers the rpm by about 300 rpm and looses about 2 or 3 mph. So I am selling the turbo.

 

[Winger Ed.]

Re: Motor height....need some expert advice

If I'm reading it right, you got a few hundred more rpm, but no increase in speed with the same prop.<br /><br />If that's true, it sounds like you've got some additional weight (maybe more fuel or water in the bilge perhaps)added on for the second run which would increase drag and slow you down at the same rpm. Or you'd need more rpm to go the same speed when the boat is lighter.<br /><br />With my little boat running a stock 150, it will turn up about 400 rpm more and get a extra couple mph when the 16 gal. tank is at 1/4 rather than being full.<br /><br />That prop sounds about right. Most guys around here with fiberglass hulls in the 17-19 ft. range use a lot of 150s/175s with a 24" p. prop. they generally run in the mid. 50s/low 60s until they start piling in the weight. Your Alum. hull should be enough lighter to over come the little bit of drag from the rivets. At speed, there isn't that many of them in the water anyway. By all rights, that boat being fairly empty- with that much engine should be in the low to mid 60's with that much horsepower. <br /><br />I'd wonder if the engine was tuned as perfectly as it could be, or the fuel pump was delivering as it should at peak demand.

 

[spego]

Re: Motor height....need some expert advice

Yeah thats what I was thinking too....should be in the 60's but it won't do it. Speed was measured with a gps and that was it.<br /><br />As far as the weight. Same 2 guys....same amount of gas....same everything down to the amount of slit shot. I dont get it either. I gained 1 mph and about 400 rpm.<br /><br />As far as the motor.....it has been gone over and over....carbs rebuilt...everything. It runs top notch believe me. You hit it down from a dead stop and it pulls out of the hole and feels like the seat is going to break....yeah it really takes off that fast. So I think the motor is running right. Maybe there is just more weight there than I think....never tried to get it weighed.<br /><br />But anyway regardless I guess I need to get a prop that will turn the rpm's in the correct range. Suggestions? I was looking at the 1/2 price sale on Iboats on the ballistic props. 14 3/8 x 21 ? or 14 1/2 x 22? These are what I was looking at but I may not be right. The diameter is different than the one I have now....14 1/4 verse 14 3/8 or 14 1/2.<br /><br />Advise?

 

[KCLOST]

Re: Motor height....need some expert advice

Are you sure the tach is working properly????<br /><br />That's pretty low rpms for that speed.<br /><br />My heavy 18ft Ranger with a Merc. 175 black max will run about 62mph at 5800 rpms with a 25p 3-blade... It will run 64mph at 5600 with a 26p 3-blade... It will also run 60-62mph at 6000+ with a 24p 4-blade trophy... I can cruise at 55mph, at around 5200rpms. <br /><br />My point is, that your speed doesn't sound too bad, but the rpms seem way too low in conjunction with the 54mph you are reaching.\<br /><br />If your hole shot is great now, you don't want to lower the prop pitch anymore... You'll just sacrifice top peed. You should be able to get up to 5300rpms with your current prop. Finding another 1000 rpms without sacrificing speed is going to be tough if not impossible by using lower pitch props.<br /><br />I'd check the tach... Is it set properly for the motor it's hooked up to...

 

[spego]

Re: Motor height....need some expert advice

Well I know that the tach works correctly up through the mid range anyway....according to the manual anyway. I would have to get it hooked up and test it to make sure the top end is correct. It does sound correct though. It really doesn't sound as if its working that hard to hit that 54 mph. Thats why all this is so confusing....I know there is more there but it acts like something is dragging and slowing it down. Maybe I should consider getting it weighed somewhere.

 

[Paul Moir]

Re: Motor height....need some expert advice

If your gearcase ratio is 1.86:1, then something's off:<br /><br />24p / 12(inchs per foot) * 4300rpm / 1.86 = 4624 ft/m<br />Which is 52.5 mph on an 'ideal' boat. (4624 / 5280(ft per mile) * 60 (mins per hour)) Real boats cause a bit of slip, figure on 5-10% for a light boat making you go around 48mph. Something has to be inaccurate, either the tach, the gearcase ratio I used, the prop pitch, or the speed. Since we know the speed to be true, and you can confirm that ratio in your owner's manual, we should be able to prove the tach wrong.

 

[spego]

Re: Motor height....need some expert advice

Paul thanks for that info. I will look it up but I am pretty sure the ratio is 1.86. So if this formula is correct then it really would be showing that the tach is reading higher than it should be or the slip is greater than 10%.<br /><br />Let's just look at this and tell me if this makes sense.<br /><br />But theoretically if you take that equation and don't mess with slipage....it is about right. So figuring that.....<br /><br />21 (different prop) / 12 * 5200 (ideal rpm) / 1.86 = 4892<br /><br />4892 / 5280 * 60 = 55.6 mph<br /><br />That is mathematically with a 21 pitch prop.....compared to the 52.5 that the current prop equation (not calculating slipage) is getting at 4300 rpm (which is all the motor can seem to turn)<br /><br />So would this sound right if I were to put a 21 pitch prop on it?

 

[spego]

Re: Motor height....need some expert advice

actually my fault.....the tach would be wrong....the slippage would be in the negatives.

 

[shep70057]

Re: Motor height....need some expert advice

I'm looking to get rid of a 14.25-20 Suzuki prop. It has too much pitch for my set up. I've run it a couple of times and has been sitting under my rear deck for a spare. If you are interested drop me a line .... asheppard@oceaneering.com

 

[spego]

Re: Motor height....need some expert advice

Yeah I checked and it is 1.8675 on the ratio. I was looking at that before and had a brain fart. So really the rpm should be 4910-5261 (would be calculating 10% to 16% slip). So it looks like th tach might be reading low. Any suggestions how to check it?

 

[spego]

Re: Motor height....need some expert advice

Might be on to something here. I looked up the tach and it has 4 6 and 12 pole setting on the back. The manual says that it may read incorrectly if the setting is wrong but never says what it should be? Anyone know what it should be set to?

 

[Winger Ed.]

Re: Motor height....need some expert advice

So really the rpm should be 4910-5261 (would be calculating 10% to 16% slip). So it looks like th tach might be reading low. Any suggestions how to check it? [/QB]

You'll just have to get a portable mechanic's tack to check it with and move the adjustor on the back of it, or swap it out.<br /><br /><br />But if your rpm is at 52-5300, you're doing about right until you start shedding some weight. From what I've seen you're running a very popular hp. of engine for that size & style of hull. It's not by accident that so many folks are using that combination.<br /><br />Leave your buddy on the beach, and run with about 15 or so gal.s of fuel and your speed & rpm should come up about 10-ish mph or so. That'll determine your best speed and give you bragging rights to it.<br /><br />You can drop the pitch of the prop a inch or two to get your rpms up with the extra weight if that's how you normally opperate the boat.<br /><br />The down side of running a alot with a prop that <br />lets you turn up much over 5500 for extended periods of time is its more prone to run hot or even overheat, and a shorter engine life.<br /><br />On my Merc. 150, the temp stays solidly at 140 until I run it more than a couple miles over 5800 or so. Then it comes right on up to 180 or more.<br />And that's with a brand new water pump and a little 'scooper' plate screwed down on the left side of the water intake holes.