B25 '68 advice

[Yannis]

These are 5 pics of a mint Riva 25 moored right next to me at this very moment.
Tell me if you need something, I’ll jump over and play Zoro!


https://flic.kr/p/2mj27u2

[beachbum]

Thanks all

Thinking more or less along the same lines as me but good to know that she might be a bit heavier, I guess this is down to the 460kg (1014lb) engine. Add fluids, the leg and transom assembly etc and it must be over 600kg (1300lb).

The chines are dead on the waterline at the aft.

The propeller needs changing, the old antifoul needs to come off (taken back to gel coat, there's loads of old antifoul on the hull) and the current prop is heavily antifouled.

Add all these factors and it is making a lot of sense.

One more question, couldn't be anything to do with a dodgy waste gate? Turbo is new but I didn't really notice any significant improvements in performance between old and new turbo, despite the old one being written off by the specialists.

[beachbum]

These are 5 pics of a mint Riva 25 moored right next to me at this very moment.
Tell me if you need something, I’ll jump over and play Zoro!


https://flic.kr/p/2mj27u2

My good god that's beautiful

[CamB25]

Dead on the waterline I take to mean they are not submerged. That's great! It's a good data point for me...I might be able to carry one of the new Merc V12s, or at least a 5.6L Yamaha. It's on my Christmas list, but I've been a bad boy this year so far, so I'm not hopeful.

Cleaning up the hull and fouled prop should help alot. I was amazed at the performance loss on my boat with just some slime on the bottom. Anti-fouling on a prop? Never seen that...sounds like a recipe for a slow boat.

Your making rpm, so I think your engine is probably fine. If you were down on horsepower you would never reach WOT.

Maybe try hiring a diver to clean the bottom and swapping out that prop for a good stainless steel unit before hauling for a bottom job. Gives you a new data point that might be enough to meet your goals.

Good Luck!

[beachbum]

Cheers Cam.

Boat is filthy underneath at the moment so it's not worth even trying until I get her out and give her a spray.

For info - I was a bit off with the chine vs waterline. Port side sits about 10mm above the waterline whereas stbd is 10mm underwater. She's always leaned a little to stbd, when I first got hold of her it was quite substantial as everything weighty was on the same side. I've since balanced the distribution a bit better so she sits nearly level. All the deck hatches drain into the stbd side hatch (which is a live bait well) which doesn't help either.

[Carl]

Your making rpm, so I think your engine is probably fine. If you were down on horsepower you would never reach WOT.

Yes and no Cam.

Yes he's making WOT, but attaining WOT rpm can be done with a bum motor by pulling pitch out of the wheel.

Pull pitch out and boat goes slower and that is where we started when beachbum seemed disappointed by the 20knt cruise. At first, it seemed kinda slow for that much hp...then a fouled bottom and wheel were mentioned. The other item I did not expect to hear is the weight of that motor...with high rpm I was expecting the motor to be a lightweight motor like Yanmar or Steyr. Damn, it's heavier than a Cummins. Deep vee and lots of weight require lots of power to push.

I think it goes back to a good prop person. One who can take all the information on hull design, weight, hp,WOT rpm, gear ratio and figure out what should be the ideal wheel and what speeds should be attainable. I'm thinking abot 25's I've played with...but they were all newer, lighter boats with modified v's and gas. Much different ballpark.

[beachbum]

Thanks Carl

Yes I found the name of a prop specialist near to me and I'll be investigating in the winter. If the rest of the boat was anything to go by, there will be 20+ years of antifoul accumulated on the hull. I simply didn't have time to take her back to the gelcoat last winter but I should do this time around.

P.S. I was lucky enough to have a trip on a 31 last week, in the Straits of Bonifacio (the small gap between the islands of Corsica and Sardinia), 5-6ft on the beam, 26-32 knots of wind. Didn't drop below 18 knots SOG. I don't think I've ever been so impressed on a boat, ever. Possibly on a Botnia Targa, but damn it was good.

Mike

[Carl]

I don't think I've ever been so impressed on a boat, ever. Possibly on a Botnia Targa, but damn it was good.

Mike

Then take into account the great classic lines of the 31, they still turn heads.

Amazing to think these boats were designed and built 60 years ago.

[Yannis]

Not to say how much better it would have been in a 28, rather than a 31. As dry as James Bond in a three piece suite.

[beachbum]

So on we go. During the time she was in the water from March 2021 to July 2022, there had been a constant drip in from the sterndrive transom assembly. I knew my repair attempt on the steering shaft hadn't worked as the area the steering seal sits in, was too far gone. New transom assembly required which means engine out etc etc all over again.

One thing I noticed during the winter 2020-2021 was the state of the hull, it needed to go back to gelcoat (or fibreglass), it was a mess. So this was going to be a fairly heavy yard period.

July 2022: I nearly had a heart attack when I found the transom to more or less peel off around the transom assembly





This was becoming very unnerving but after all was removed, chipped, grinded away, what was left was actually in good condition. I had a surveyor check the moisture readings, which were slightly high but nothing crazy. She was about to sit in 35°c heat for 2 months so I was confident. Unfortunately the down side was that stripping the hull was nearly unbearable. I had to start at 5.30am and by 9am, stop as it was just impossible, sweat just pouring off me (as I was in a nuclear style protective suit, goggles, mask, gloves..)

Started like this:


Almost finished:


After cleaning every last speck of old cr*p off the bottom, it was time for 4 coats of epoxy treatment.


Then moved her inside and put the new transom assembly in:


Launch day, 11th Jan, after another 6 months out of the water:


And here she is again. No leaks, ran well, manages about 1 knot more than she did at the last launch so the hull work did have some effect.


When in the strops being launched, I got a readout of the weight, approx 3100 kgs (6800lbs). This doesn't seem crazy heavy so I am still disappointed with the 26.5 knots WOT, but I think a smaller diameter / larger pitch prop might be the way forward, as I was expecting it to be heavier than this.

25' guys, what are you weighing in at?

[captbone]

Looks good.

What engine are you running currently? What ratio is your outdrive and what size propeller are you currently running?

[beachbum]

I've tried measuring the ratio myself and I think it's 1.65, the prop I'm not sure but looks to be a 21" diameter

[captbone]

Post a picture of the prop and outdrive if you have it. 21 inches in diameter does not sound right. 21 pitch?

[beachbum]

Thanks captbone - these are the best I've got. Engine is a 4.2 litre Mercruiser diesel, 320hp. 3900rpm with the limiter at 4000, she reaches 4000rpm no problem, and can do so even when a bit dirty.

[Ironworker]

You've done a great job with that boat!

[captbone]

Looks like a Left hand rotation Bravo 2 sterndrive.

For speed you can try a Bravo 1 or Bravo 3 lower unit as the uppers are all the same.

[beachbum]

Yeah that's it, it's a B2 alright. It's definitely geared up to spin free with less load and I'm sure there are another 5 knots available if I can get the leg set up properly.

I've looked at the 3X although it's going to be out of the question for the moment ($€£) - 2nd hand units here in Europe just don't seem to exist.

That's interesting about just changing the lower unit though. I get scared giving sterndrives too much torque to deal with but if the upper gearset is good then there is no reason to change it. Something I will look in to, thanks for the tip.

[beachbum]

Hey all,

So nearly 2 years down the line, I'm still going strong with the '25.

However I've got a dilema which I know someone here will have the knowledge to advise on.

My '25 has a 320hp diesel coupled to a Bravo 2 leg. After much scraping and cleaning, I finally found the all important data I was looking for.

Ratio 2.2:1, prop pitch 21 inch, prop diameter 17.75 inch. This gives about 26 knots, but the engine, which should hit 3900rpm, gets to the limiter @ 4000 no problem at all before I've even got the lever all the way down. I estimate it'd get to 4200 if it were left to go unlimited.

My dilema: the prop options for a Bravo 2 are limited. I can go to the next step up which is 23 inch pitch / 17.5 diameter, or (as I think the boat is so underpropped), go to a 25 inch pitch / 17.25 inch diameter. My basic calculations would suggest 4" extra pitch would lose 600rpm off WOT, but then the 0.5 inch less diameter would gain about 300 rpm (?), net result is 300rpm less which I think would bring me into line at 3900 rpm.

Is +4" pitch / -0.5 inch dia too much in one hit?

[Yannis]

If the guy who will sell you the new prop has a spare of (+4-0,5) that you can borrow, then you answer all your questions!

[Carl]

Rule of thumb- An inch of pitch is 200 rpm.

4200 to 3900 is 300rpm to play with. It is advisable for motor to attain 50-100 rpm over rated which gives you 200 rpm to play with.

I’d start by increasing pitch 1” giving 4000. Maybe add some cup to knock off another 100 rpm. If you think motor has more to go. What’s good about cup, if too much it can me knocked out easier then a pitch change.


BUT what Yannis said about borrowing a prop is the best suggestion to dial in

[beachbum]

Thanks everyone. I can get a 25p x 17.25 dia to try to free.

I'll give it a go and if it doesn't reach revs then I suppose I will know if a 23p is going to work or not, i.e. if the 25p is miles away from making 4000 then there isn't much point in trying the 23 either!

[JeremyD]

I would assume the gear ratio is the culprit. On gas engines that can spool up to 5,000 rpm's most transmissions are set up with a 1:1 gear ratio

With a diesel - You typically want a higher gear ratio - at 1:1.65 at 3900 rpms you are spinning the prop at 6435 RPMs - should be more than enough to get you over 25 miles per hour.

Compatible drives for the 4.2

Bravo One XR
Bravo Three XR 1.81
Bravo Two XR

You have to be careful with outdrives - the 4.2 produces 577 foot pounds of torque - easy to blow one of the gas versions to pieces. With the Bravo 3 gives you counter rotating props. definitely more expensive, but also a lot more bite.

[Carl]

Is +4" pitch / -0.5 inch dia too much in one hit?

I think it is too much in one hit as you do not know how far over the 4000 the motor will go. That said, you have an opportunity to find out without costing you the cost of the prop, so the Hit is avoided. It cost the time it takes to change prop or 2 hauls if you cannot do in water.

If motor is actually capable of going past the 4200 your assuming it could hit...then that extra 4" pitch (-1/2" dia) may work like a charm. If you wind up only hitting 3800 or 3700 you have good numbers for your prop shop to work with. I'd recommend not running motor very long if that is the case

Good luck, keep us posted.

[beachbum]

Thanks all.

I have managed to unearth the manual for this drive, or part of it at least. The drive is from 1996, at which point there was no "X" or diesel version of Mercruiser sterndrives. A very interesting point in the manual:

"Normally, a 150rpm change exists for every 2 in. change in propeller pitch when using Quicksilver products"

Which I suppose answers the "what difference does the diameter make?" question - quite a big one, it would seem, if 2 inches of pitch change only alters the engine rpm by 150.

[Carl]

A very interesting point in the manual:

"Normally, a 150rpm change exists for every 2 in. change in propeller pitch when using Quicksilver products"

That is very interesting. I'd love to find out why Quicksilver products vary from others. in any case...if there is a manual, I'd use that over Rule of Thumb.

[Yannis]

Carl, a 50 rpm difference falls well within the rule of thumb!

[Carl]

Carl, a 50 rpm difference falls well within the rule of thumb!

Its 125rpm difference.

Rule of thumb I go by is 1"pitch change = 200rpm. BeachBum is stated 2"pitch change is 150rpm difference according to Quicksilver.

[Yannis]

Carl,

I was carried away by this bizarre system you have with inches and stuff!
Besides, who said that Math was my strong point?
Thanx.

[Carl]

Yannis- I work in the inch system every day for hours on end and still need to look up fractional to inch decimal conversions. Arbitrary sizes based on nothing. 12 inches in a foot and 3 feet in a yard, but 5280 feet in a mile? why not measure in yards at 1760 yards, we can, but don't, not that is matters, it's all just stupid.