What performance to expect from F-25?

wet wonder · Post by **wet wonder** » Wed May 06, 2009 5:56 pm

I'm trying to get our 1975 F-25 to the right performance. It has a '77 Mercruiser 351 233hp, and I just put on a 13x10 prop, which improved the rpms. After switching out the prop, I took it out on calm water and got WOT at 4,000 rpm and about 16 knots on my GPS at best trim (18.5 mph). I was hoping that fully open it would get 18 to 20 knots, and that I could achieve a 15 knot cruising speed. I'll be swapping out plugs and checking the timing this weekend.

Anyone out there have or know about this config? - What kind of performance should I expect, what RPM WOT, what cruising rpm and speed?

jwrape · Post by **jwrape** » Thu May 07, 2009 7:59 am

I know that Electronic ignition woke up my twin Chrysler 440's.... It's REALLY one of the best mods for these old boats.

randyp · Post by **randyp** » Thu May 07, 2009 8:56 am

The mighty Blue Heron can get up to 4200 RPM WOT and that's really scary....got up to 24 mph (GPS) with a light load, clean bottom, and half tank of gas in flat water. Those are the upper limits for the F26 with a Chrysler 318 225 hp). You should be getting over 20 mph, but not sure of your bottom condition and loading. I have the same prop.

wet wonder · Post by **wet wonder** » Thu May 07, 2009 11:39 pm

Thanks a lot for your reply Randy. That's very helpful. I'm at about 18.5 mph now, and about 3,900 rpms. This weekend I'm going to check the fuel filter and fuel system, clean out the spark arrester, change the plugs and check the timing. I'm hoping that all this will give Wet Wonder a little more performance and bring the speed closer to spec.

Incidentally, I saw on the trojan catalogues that the F-25 curve is 14 knots at 2500 rpms, 20 knots at 3000rpms, and 28 knots at 4000 rpms, so that's the expected baseline that I'm using. Somehow, I'm not reaching the hp potential of this motor.

captainmaniac · Post by **captainmaniac** » Fri May 08, 2009 1:05 am

Trojan's published specs for a 1979 F25 with 225 Chrysler or 228 Mercruiser reads :
- low cruise at 2500 RPM, 16.5 mph
- max cruise at 3000 RPM, 23.3 mph
- top end at 4000 RPM, 32.0 mph

... but I think their numbers are a bit optimistic.... My F32 is spec'd out at 34.5 mph, but on the two occasions I have opened it up (never for more than a couple of minutes, to make a bridge !) I have never gone beyond 30...

kevin babineau · Post by **kevin babineau** » Fri May 08, 2009 7:57 am

where do u get these specs? and are there any on the f36?

prowlersfish · Post by **prowlersfish** » Fri May 08, 2009 1:14 pm

you can change the plugs and set the timing hell you can put a turbo on it your still going to get 18.5 at 3900 rpm .

check the tach find out what the rpm is for sure you may need a liltle more prop , maybe your engine not realy geting that rpm . untill you know , you can't know how to fix it .

wet wonder · Post by **wet wonder** » Fri May 08, 2009 1:52 pm

The specs were on the catalog page image that's on this website.

wet wonder · Post by **wet wonder** » Fri May 08, 2009 1:57 pm

prowlersfish wrote:you can change the plugs and set the timing hell you can put a turbo on it your still going to get 18.5 at 3900 rpm .

check the tach find out what the rpm is for sure you may need a liltle more prop , maybe your engine not realy geting that rpm . untill you know , you can't know how to fix it .

Yeah, I know. But when I had the larger 14x10 prop, I could only achieve 3k rpm. I've communicated with folks that had these same symptoms, and they had a few different fixes. (1) timing was off, timing correction fixed the problem. (2) fuel filter was clogged, cleaning it fixed the problem. (3) Carb problem, rebuild or correction fixed the problem. So I'm just going to do the rule out process and check off each thing one at a time till I find the problem.

prowlersfish · Post by **prowlersfish** » Fri May 08, 2009 4:54 pm

Your would not think 1' of prop would make a 900 rpm diffrence . of course they could have been reworked to diffrent specs

cookscomp · Post by **cookscomp** » Fri May 08, 2009 7:01 pm

My f-26 has the same 233hp mercruiser with the electronic ignition conversion done. It also has a 14x10 prop. Unfortunately Ive only been out with it for sea-trial last fall, but gps confirmed 24mph. Sorry didn't notice the rpms. I don't see us going over that speed range except maybe once or twice a season to confirm its still capable.

prowlersfish · Post by **prowlersfish** » Fri May 08, 2009 7:14 pm

wet wonder we know your boat ran like it was over proped with the 14X10 could some one have have cup added to it ( or can with a lot of cup ) or added some pitch Like I said 1' more prop should not drop your rpm 900-1000 . some prop shops will check that for free . you may have more then one proplem ( its a boat ) . if you don't get it sorted by the Rendezvous I will glad to look at it for you ( free ) I have done this for a living . Its a lot easier hands on then on line thats for sure

MattSC · Post by **MattSC** » Fri May 08, 2009 9:03 pm

After I repowered, I tried several prop combinations for my F-26 and learned some things that my boat liked and didn't like. At first we tried a 14 x 11 with no cup. I reached the recommended rpm for the engine, just under 5100rpm, but the most I could get speedwise was around 22-23knots and it seemed like the propeller was "slipping" in the water. I had a degree taken out of the prop and added a full cup. I now reach the same rpm, the boat can reach 31 knots at WOT and a 17-18 Knot cruise at 3000 rpm. I never thought it would have made such a difference but it did. I'm not saying this might be the same for different models, but thats the setup that works for my F-26. It has a 5.7 MPI Crusader 330 HP and a Borg Warner 1:1 transmission.

jon_e_quest · Post by **jon_e_quest** » Sat May 09, 2009 3:22 pm

I owned a '78 F-25 HT with a Chrysler 225, and 14X10 cupped wheel. The boat topped out at 4K rpm indicated to achieve 28 mph GPS (half tanks).