TOPIC: More top end from my 74' 850??

Here's what I'm getting now. 17 for the pitch (have to look at diameter tonight), at WOT RPM's are at @5200 and GPS speed @ 30 to 34 MPH. Now max RPM range is 5500 so what prop size should I look at to get more top end with out going over that 5500 RPM point?? Everyone that has seen my 18 ft silverline says I should be able to get at least 40 to 45 MPH out of it with the 85 on it and they seemed shocked when I tell them only 30 to 34 MPH.

SS

Suncrest Sniper wrote:

Here's what I'm getting now. 17 for the pitch (have to look at diameter tonight), at WOT RPM's are at @5200 and GPS speed @ 30 to 34 MPH. Now max RPM range is 5500 so what prop size should I look at to get more top end with out going over that 5500 RPM point?? Everyone that has seen my 18 ft silverline says I should be able to get at least 40 to 45 MPH out of it with the 85 on it and they seemed shocked when I tell them only 30 to 34 MPH.

SS

Hate to be the bearer of bad news BUT...

40-45 on an 18 foot boat with 85HP just isn't going to happen...unless maybe the entire boat was gutted and you were running the motor VERY high on a jack plate with a fair amount of setback and a specialized prop. You're within the recommended WOT RPM range and changing props really will not gain you much of anything, top end wise(maybe another MPH or two at best). Dropping down to a 15P prop will get your RPM's into the 5500-5600 range but you will likely lose a couple, few MPH. It will come out of the hole better though.

30-34 is plenty respectable for your setup. Really the only way you are going to get significantly faster is to put a larger motor in its place.

If you're currently running an aluminum prop, you may be able to gain some rpm at the top end by going to a 17" stainless steel prop. The thinner blades will give you a better hole shot and spin up deeper into the motor's power band.

Note if you're already running a stainless prop, you're pretty well maxed out unless you can find a prop of equivalent pitch that will spin up a bit higher. Every prop, even of the same pitch, will be just a bit different.

It won't hurt this motor to spin it up to around 5800 or so but it ain't necessarily gonna make more hp. Just depends on when it runs out of 'steam' at the top end.

Another area of potential gains is motor height on the transom. If your anti-ventilation plate is even with the bottom of the keel, you may gain speed by raising the motor as much as 3/4"-1". Note that gains are more pronounced on hi-performance hulls, yours is 'not' and YMMV. 1/4"-3/8" maple or other hardwood spacer shims work well for height experimentation.

BTW you should probably do a "link & synch" on your motor to make sure throttle butterflies are properly synchronized and fully opened at max throttle. Also check maximum spark advance, DO NOT set at the sticker value of 27 deg BTDC since this is too much for today's fuels. 21-23 deg is a better setting and you'll have less tendency to burn pistons!

Other than the above, not much you're gonna do to make 'er go faster unless you drop an Inline Six powerhead on the 85hp mid! And you'll need to change the exhaust tuner plate as well, since the 4-cyl tuner is too small & restrictive for a Six.

Here's some good info from "Clams Canino" regarding Link & Synch of your motor:

forums.iboats.com/showthread.php?t=168855

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

15p would be a good idea. Mid- 30 mph is good for an 18' heavy boat. Keep in mind, your 85 on a GT-150 (650lbs) runs 50mph stock. I'd bet your boat is 1000+ lbs. or so.

Great info guys. It looks like I may just leave it the way it is. My hole shot is great. She leaps up onto plane very fast and the ride is smooth so it appears it's as good as it's gonna get. Over the winter I want to try to locate a 100 to 150 for it and put the 85 on my old Biesemeyer Flat Bottom project.

Thanks much,

SS

Something worth mentioning about the 850 4 cyl that may not apply in this specific case, since you are reporting a good hole shot with the 17p, is that originally the 850 came out with a 2.31:1 gear ratio lower unit, for more low end power and consequently less top end speed.
In the past (37) years it is always possible that the original lower unit may have been replaced with one off an inline-6, which fits perfectly, but has a 2:1 ratio.
The 850 4 cyl simply is not 'happy' with a 2:1 on any average to heavy hull.
To determine which gear ratio you or anyone else might have the following test will let you know.
1) Turn off the ignition key and leave the external harness attached to insure the beast doesn't try and start while doing the test.
2) Put the controller in Forward gear.
3) Put a 15/16" socket and flex-bar in place on the flywheel nut and rotate the flywheel clockwise until the propshaft begins to turn with the crankshaft.
4) Stop rotating and put a straight mark on the gearcase and onto the prop or propshaft.
5) Put a mark on the flywheel and onto the cowl support at the edge of the flywheel.
6) Rotate the flywheel clockwise while keeping track of how many rotations the flywheel needs to complete until the line on the gear case & prop or propshaft align again.
7) If the flywheel completes (2) turns the gear case is a 2:1.
8) If the flywheel completes about 2 1/3 turns the gear case is a 2.31:1.
Some smaller Mercs have a variety of other gear ratios and this procedure will also help identify if they still have the original ratio that Merc concluded was best fit for the horse power & torque produced by the specific Merc.
Thom