Twin Mercury 200 2.5 offshore props and revs.

tomshep · 01-10-2010, 06:03 PM

Hi there. I recently put up post about my Ring 24.
I need some advice on revs for the 200 mercs.What revs should they pull at top speed.
I have rh+lh rotation laser 2. 23" pitch. it goes 75 mph at 5700. I think there should be more, it doesn't sound like there reving enough any ideas??
Also does any one know, what props would be better suited to this type of set up.

Ben · 01-10-2010, 06:36 PM

Year and model for the engines?

tomshep · 01-10-2010, 06:48 PM

1991 2.5 offshore 200 hp,

cfun · 01-10-2010, 06:57 PM

That boat used to hang out down my boatyard in the 90's chatted a few times with them seem to recall they did that exhaust as they claimed it gave a few more HP and sounded a lot better, but I am sure they only achieved mid 60's to late 60's with it after many years of playing with it

tomshep · 01-10-2010, 06:59 PM

well this does 75 on gps, but its not reving enough, only 5700. whats the limits. any ideas??
The exhausts, do make it sound better I think. It was doing 65 mph with the 27" mirages but wanted better. so went for the lasers. Dont know if there would be any better ones or not maybe surface piercing or some 4 blade props, I dont know enough about props to do it, Also props are bloody expencive so dont want to throw away too much cash.

cfun · 01-10-2010, 07:09 PM

Not saying it doesn't ...
however the speed does not stack up with a 23'' prop and pulling 57K, having been on and run single engine boats with 200's on 21' Ring's and Phantoms even with 6K+ and on a worked 26'' prop speeds were still in the 60's with the standard gear ratio, my present very light boat with an XR2 only did 60mph at 57K with a 23'' 5 blade prop on a 1.87 gear ratio

tomshep · 01-10-2010, 07:13 PM

Any Ideas about the revs though. Where is the cut off point to blowing them up.??

gar163y · 01-10-2010, 07:23 PM

Well, for what it's worth, my guess would be a pair of these. They only go down as far as 26" so you may need to lose some pitch.

http://www.mercuryracing.com/propell...adecleaver.php

Why are there no rear tie-bars and are the mounts solid?

tomshep · 01-10-2010, 07:28 PM

Rear tie bars that's next for me to make I was thinking about that just the other day! Is it necessary to have, what will it do apart from keeping the engines apart. No there the normal mounts on the engine.
Thanks for the link to that website.Think ill order a pair on monday.

gar163y · 01-10-2010, 07:36 PM

You need to tighten everything up before you can drive the boat hard enough to find out what props you really need.
The advantage of the four bladers is that it will make the Ring corner much better and the extra grip also gives good pull out.

I ran those props in 28's on my Ring when it was a twin but it was a smaller hull with a few more horses.

tomshep · 01-10-2010, 07:38 PM

Ok, thanks, thats something to think about, Ill have a look at the cleavers. Cheers.

Burty · 01-10-2010, 08:45 PM

Do not rev them above 6k unless the oil injection has been removed!!!! Once that's been done then high 6's shouldn't be a problem.

Ben · 01-10-2010, 09:05 PM

Quote:

Originally Posted by tomshep

its not reving enough, only 5700. whats the limits. any ideas??

According to the 1991 Mercury brochure, those engines have a max rpm of 5,600. Perhaps the modified exhaust added another few revs.

Not to doubt you but the performance numbers you quoted don't seem to add up. i.e. 75mph at 5,700 rpm with 23s.

The fastest that setup can go assuming a gear ratio of 1.87 - 1 (which was the standard ratio for that motor) is 66.58 mph and that is the theoretical top speed giving no consideration to prop slip.

Easy to work out - max revs x prop pitch divided by gear ratio x .00094976 = theoretical top speed. Prop slip is typically 10-15% on a good vee hull.

Example using your original 27 in props and assuming they can be turned at 5,700 revs.... 5700 x 27/1.87 x ,000994976 = 82.3 mph. Subtract say 12% for slip and you arrive at 72.4 mph

If the engines have been modified for higher revs or have had different gear ratios installed, these numbers will of course be different.

gonein60seconds · 01-10-2010, 09:23 PM

isn't 5,700rpm the upper end of the power-band on that model?

maz928 · 01-10-2010, 10:08 PM

if its still got the standard reeds(metal) fitted it wont rev any higher than 5800/6000.

tomshep · 03-10-2010, 06:18 AM

hi ben. The reason for not keeping the 27 " mirage on the boat was because it was only pulling 4300 rpm and only going 60 mph. I couldnt get it to rev any higher, I think that was because the props that I borrowed came off a Scarab with twin 500 hps, and they were a bit to tall for my boat. The 23 lasers are good, they brought the revs up to 5900. I think like a few others have said, Ill go for the cleavers, then I may see some more speed. I like going fast you see.