xpress h51 prop

[arxpress]

I just bought a 2003 xpress h51 w/ 90 h.p. merc. The boat was a demo, i don't think it's been in the water ten times. I took it out the other day , the tach read 6600 at wot, speedo was 50 mph. 13x19 michigan ss prop. the boat is 17' long with a 78" beam,weighs around 900 pounds dry. WOT should be 5500 rpm. My question is is my tach wrong or my prop. If it's the prop would a 22 or 23 get me down to 5500.:%:%

 

[Rudy Brown]

Re: xpress h51 prop

First off, nice boat. I think your tach is off. Might want to check the setting on the rear of it. My bass boat has a 90 hp rude on it with a SS 13x19 prop and at 5800 rpms I'm getting 49 mph. At 6600rpms you should be much faster. There are several prop slip calculators on the web, search for one and plug your numbers in. You should get around 10-15% slip.

 

[Rudy Brown]

Re: xpress h51 prop

Try this http://go-fast.com/Prop_Slip_Calculator.htm

 

[walleyehed]

Re: xpress h51 prop

Make darn sure that isn't the 2.33?:1 ratio...Texasmark has a 90 too and it takes a larger pitch than the johnrude engines of the same HP because they turn slower...
Check on the stats of the gear ratio and post back...you may be too skinny on pitch.
Texasmark, where ya at?

 

[arxpress]

Re: xpress h51 prop

I loaded it down with all my stuff and another person yesterday, rpm's read 6200 wot at 45 mph. speedo. The gear ratio is 2.33. The tach is a faria, the back of it says for a 12 pole set to # 5, that is what it is set on. I have heard these boats,with a 90 merc, run better with a 13 1/2 x 22 laser 2 prop. I ordered one yesterday, should have it in about a week.

 

[Rudy Brown]

Re: xpress h51 prop

I calculated 6% slip.( thats pretty good) Sounds like Kenny, and you were right and you are too skinny on prop. I'll let him comment on the prop ya chose as he's the guru d

 

[walleyehed]

Re: xpress h51 prop

Faria on #5 is correct.
The Laser will treat you fine...let us know how your RPM comes out.

 

[Texasmark]

Re: xpress h51 prop

Not familiar with the hull on that boat. From the data it seems to be an efficient design.

Your numbers look good to me for the prop pitch you are running with that engine.....know nothing else but what I have read in your thread. What I am saying here is that if I had your prop on my boat it would run where you are running.

As Kenny said, the gearbox is 2.33 and has been on the 90 for a long time......at least back thru '94. I think the 90 was geared to be a midrange workhorse which is surely is. On a light fast boat you need to prop it up.

I know setup and hull design and all that has a tremendous effect on performance as D and Ken point out.

In my case I am running a stock alum pad bass boat with a 20 degree dead rise to the transom (pic on my Avatar at 50 mph). That puts my lower unit 13" behind the bottom at the transom. The way the transom is made, the engine has to sit in the first hole up. So, my numbers do have some performance goodies built in. Don't know what you have in that arena that you might tweak before hitting the prop (got it from the pro's d . One problem I do have is that my transom tilt is so high that I run out of trim before I can get the prop to ventilate. I think if I had 5 ish degrees more trim I could get more perf out of this rig/setup.

The other thing that I found is that prop efficiency has a whopping big effect on rpm's for a given speed.....like on mine, 3 mph and 600 rpm's lost with a (an inefficient) 23P as compared to the HiPerf 24P I normally run. ........(you'd think a lower pitch would spin up faster.....not so).

I use BAM prop calculator and slip is very low....around 10%.

In summary, your gearbox is 15% lower than "normal" so for the same perf, you prop has to be 15% higher as a rule of thumb.

Per Kenny's comment, Rudy is probably running a 2:1 on his Rude and you can't go by his numbers.

HTH

Mark

 

[arxpress]

Re: xpress h51 prop

Got to try out the laser 2 prop today, man what a difference. 5200 rpm at 50 mph speedo. dropped me down 1000 rpm's. I've got some more questions. If i raise the motor up to the next hole, will it make it easier to steer and how many rpm's will it gain. Steers really hard right now. I would like to get up to 5500. It porpoises a little, will raising the motor make that worse? Center of propshaft is currently 6 1/2 inches below the bottom of boat. Cavitation plate is 3/4 inch above bottom of boat. Also what brand and pitch aluminum prop do i need as a back up.

 

[Texasmark]

Re: xpress h51 prop

I am not an expert. I have just my few life's experiences. On one boat in particular I was running a 3 cyl 70 hp Rude on a Kingfisher fiberglass fishing boat....real popular KF boat for a lotta years....bottom is essentially flat with little to no V at the bow....very similar (but lighter) to the early Ranger Bass Boat. where it was a long square looking thing with the same type hull.

I raised the engine 1 hole and I noticed that the steering was easier and also that the trim had less effect on bow lift. I don't know where the vent plate was initially and don't remember where it was after. I just remember being able to move the bow up and down (noticeably) before and after it was less prominent.

But I did gain speed. Don't remember how much but it was noticeable.

12P on the speedo is correct.

Have you tweaked your trim tab?

Depending on the hull design, raising the engine should reduce your tendency to porpose. (I'm basing this on an assumption that the vent plate is in the water now and if you jack it up it will be out. Being out of the water reduces the water deflection off the plate which should reduce bow lift for a given speed and reduce porposing. What you loose in more wetted hull with this you gain in less lower unit drag......I think .....I think but have no proof ....I know what it does cause mine did it, just what's the mechanism causing it.)

But it's going to raise your rpm's back up and I know that will really tick you off.....beins a propotional speed increase will follow 8)

HTH

Mark

 

[arxpress]

Re: xpress h51 prop

yes i have adjusted trim tab, didn't seem to help much. what is the best way to lift or support engine to raise it up one hole.

 

[Texasmark]

Re: xpress h51 prop

I now have a roll around engine hoist, like you would use to pull a car engine. Made an adapter to screw on top of the crankshaft. Best solution if you don't have a gantry crane.,

A dangerous way to do it if alone, is to use the trailer's tongue jack if you are just talking about moving up a hole; not complete removal. If you had a couple of neighbors to steady it while you cranked, and they replaced the bolts (or vice versa) when the holes line up, should work great.

Remember these engines run 300 or so #. If you have them balanced on the skeg you are ok. But a few degrees off either way and they can get away from you fast.

Good Luck,

Mark

 

[arxpress]

Re: xpress h51 prop

Found a used 6" manual jackplate, installed it yesterday, tested it today. I think it's set up pretty good now. Gained 3 mph and 300 rpm's. Currently running 5500 rpm at 53 mph speedo. Hard steering and porpoising are gone. Center of propshaft is 5 1/2" below bottom of boat. Thanks for everyone's help.

 

[jamieat]

Re: xpress h51 prop

I have a question for you. I have the exact same boat, and H51 with a 90 HP Yamaha, 2004 model. I have a 6" jack plate and am currently running a 13.25 x 19 pitch Hustler vented 3 blade prop. I can get about 5800 RPM max with two people and gear. I have always had problems with the boat porpoising when there are two people in the boat. The jack plate and the prop helped a good bit, but the boat still porpoises when it is trimed up. Strange thing though, is that if I am turning just a slight bit to the left, I can trim the boat to the max, and hit max speed and rpm with zero porpoising. As soon as I straighten the boat up, it will porpoise terribly. With just one person in the boat, I can trim it up to max and go left, right, or straight with zero porpoising. This has become a real pain in the .... for me b/c I typically fish with a partner. Please help, any advice would be greatly appreciated.