Should I change ratios in my Volvo AQ290?

[Reflexx175]

I just finished a 27 month restoration of a 1967 Donzi Ski-Sporter. In 1987 the original engine/drive was replaced with a 310hp Volvo AQ290 with trim. 1.61:1 outdrive ratio, verified by rotating the coupler and watching/counting prop shaft revolutions.

I had the engine rebuilt (.030 over, has Holly 715 single pump, vacuum secondaries 4-bbl, GM Dual Plane intake, OSCO center riser exhaust, Mercruiser Thunderbolt IV ignition/distributor, Mellings 22124 cam).

I have run a Solas Titan 15-5/8" x 19" prop (50.8 gps @ 4,800 rpms), a Volvo aluminum 14" x 23" (53.8 gps mph @ 4,800 rpms) and a Volvo Ultra 14" x 24" (53.8 gps mph @ 4,800). Everything I have read states that this vintage Thunderbolt IV ignition doe NOT have a rev-limiter in it. I agree. I can run the engine up to over 5,000 in neutral.

So, for whatever reason, this engine appears to "like" a WOT of 4,800. This is my 17th boat, and I am at a loss to explain how the WOT rpms could be the same for a 19" prop as for a 24" prop (both stainless). The Solas has virtually NO slip using my actual numbers in a prop calculator, and the Ultra has 20% slip, but all my boating experience says for every inch of increased pitch, you lose between 100 - 200 rpms. These two props should be at least 500 rpms different, and yet they both run to 4,800, as does the aluminum Volvo 14" x 23".

I would like to have a better cruise to rpm ratio. With the Ultra, 3,300 rpms is about 40 mph. That is almost acceptable. Using the Solas, 3,700 rpms is about 40, and that is spinning too fast for my liking.

Given the limited props available for the Volvo 270/280/290 outdrives, I was thinking maybe a lower ratio gear set might allow me to run my existing prop, spinning them faster at a given rpm, and thereby getting a better cruise/rpm relationship. I have no idea what the next lower (1.43:1?) ratio is, what would be involved with changing the gears, or what the cost would be.

Help? Suggestions?

 

[Bondo]

Re: Should I change ratios in my Volvo AQ290?

Ayuh,... It sounds like ya got the Right ratio now....

If ya lower the rpms for cruise, it'll be lower across the board, which will be luggin' the motor, All the time...

 

[Don S]

Re: Should I change ratios in my Volvo AQ290?

If you have 4800 rpm for WOT, what is the problem. That is exactly where you should be. Prop size only sets WOT not cruise rpm.

 

[Reflexx175]

Re: Should I change ratios in my Volvo AQ290?

The way the engine pulls, it feels like it would also spin a 26" prop to 4,800. I have not found any LH 26" props, so I was thinking a gear change would effectively increase the pitch of my current props. You know what I mean, I realize the prop pitch doesn't change. I'm just baffled that a 19" and 24", both being stainless props, both spin up to the same WOT rpm.

I feel like I'm leaving something on the table, until I find a prop that the engine won't spin to 4,800.

I realize the prop is a determining factor for what WOT rpms will be. As it stands, I have (2) different props, that are 5" different in pitch, that both rev to 4,800 rpms. The speed produced at 3,500 rpms is about 6 mph different, with the 24 being faster. Say I could spin a 26" also to 4,800 (I know it shouldn't happen, but neither should my current 19/24 happen), it would be faster at 3,500 rpms, giving me a better cruise.

Most of the people I cruise with (other Donzi classic 16, 18, Minx and 22) cruise between 40 and 45 when we are running. In my current set up, to run with them I am between 3,600 and 4,000 rpms, depending on which prop. I'm not comfortable doing extended running of the engine at 4,000. Maybe I shouldn't worry about that, and the engine will be fine. The secondaries (vacuum) aren't open at 4,000 so it should be fairly efficient.

I'm just seeking information on what my options might be. I thought re-gearing the lower might be an option, given that I haven't found a prop with enough pitch to give me a WOT less than 4,800.

 

[Tail_Gunner]

Re: Should I change ratios in my Volvo AQ290?

Lot of power there in a small boat..If you change the gearing to a 1.5 or even a 1.4 you can use a smaller pitched prop for launch..Looking at your numbers you seem to have plenty of touqe at the top end of the rpm range. Do you know the cams specs..It would be nice to know.

I guess if you have deep pockets and want to tinker yes changing the pitch will allow you a better hole shot and increased top end. It is as simple as smaller pitch launches the boat and at the same time less prop area at the top end reduces drag and at 50 or above it can be significant

Generally speaking 18' of boat and v6 200hp is geared with a 1.66 from there 1.5 ratios are used in v8 application's they do have more touqre at the top of the rpm band

The cam ratio is significant however...And where it makes power along with your boating preferences....aka if you run on small bodys of water it doesnt make to much sense to setup a boat to run well for a 5 min hop and your done or tube and ski alot.. However if you cruise for 20minutes to a hour well... personally i would change...But thats just a opinion

Little side note...Use GPS to measure speed and verify that tach is correct within 100 rpm across the rpm range if your going chase this around...Boat tachs look pretty and they are rugged.....But they lie..lie lie....

 

[Reflexx175]

Re: Should I change ratios in my Volvo AQ290?

I boat a LOT in Lake Michigan, so much of my boating consists of 15 - 60 mile runs.

I believe the ratios you reference are Mercruiser. Volvo's are slightly different, but I get what you are saying.

My camshaft is a Melling part # 22124, which is the same camshaft used in the Mercury 350 engines from 260 to 320 hp (intake manifold, carb/fuel injected differ). The Melling tech support person said, "Peak torque on this marine cam will be @ approx.. 2600 RPM with a power range of 1000 to 4500."

I used my Garmin NUVI GPS for the speed numbers given. The tach is a new Faria 0 - 6000 rpm tach, and while it could be lying, I would think it would be consistent with itself, so even if it is off 300 rpms, it will always be off 300 rpms, making relative comparisons between different props valid.

 

[Tail_Gunner]

Re: Should I change ratios in my Volvo AQ290?

From what ive seen on that cam it is a realtivley mild cam, the fact that you can turn 5000 with 5" of pitch differnce shows as much..aka the engine is out of air and lots of tourqe up to that point. Again if the boat was mine yes i would change gearing and as you probably already know that will allow smaller pitched props what will launch better and give higher top end (less drag)...It is a light boat with a deep v and looks fun nice job on the restore

Older Donz's seem to do well with Solas props...increasing your midrange performance is 4 blade area A Stilletto four blade really shines in that area. However they have huge lift and with a sharp v chine walking can come into play quickly above fifty..Good luck your project looks fun

 

[craze1cars]

Re: Should I change ratios in my Volvo AQ290?

I guess what comes to mind is the fact that you're comparing a Volvo Ultra prop (which has been out of production for how many decades now??) to a modern Solas prop.

Despite having a 5" rated pitch difference, the cupping/raking/efficiency/blade designs between the two must be drastically different. Great advances have been made in propeller technology since that Ultra was popular....and I think your slip calculations confirm that.

If I were you, and I wanted to tinker/toy with a little bit of money, I'd try some bigger pitched Solas props (and I'm only suggesting Solas because it happens to be what you have right now in a 19....trying for apples-to-apples comparison here, which you do not have yet.) So try the SAME design as your 19 p Solas in a 21, or maybe a 23. I think you might be surprised to start seeing your RPMS crawl down a bit more like you would expect. And maybe if you motor is indeed running out of air at 4,800 it can actually push those bigger props at a similar RPM...which would be GREAT because you'd be going faster in the process!! Plus your cruise RPM would come down substantailly, which is one of your objectives. Your very own math shows that the Solas is the most efficient prop of the bunch by far...so why not work with that? Since you're only at a 19 now and you have plenty of room to grow from there...if it works...

I'd certainly try out a few bigger pitched, stainless, MODERN props before going thru the expense of a regear...which ALSO might not work...and is FAR more expensive and difficult to reverse than it is to just sell a prop that didn't work.

You might be surprised at how much better a new modern stainless prop is than an old Ultra....and then again you might not....

 

[Tail_Gunner]

Re: Should I change ratios in my Volvo AQ290?

...donno how that happened

 

[Maclin]

Re: Should I change ratios in my Volvo AQ290?

Have you ever thought about the swap to DuoProps. I believe it is just the lower end swap, do not quote me. The DuoProps will holeshot better, maintain a lower planing speed, maintain a higher crusing speed and give the same topend. Lots of props available for those everywhere too. Just sayin' it's only money. Ok, only YOUR money....


They do make a world of difference.

 

[Reflexx175]

Re: Should I change ratios in my Volvo AQ290?

Craze1cars, I think you are absolutely right about trying a steeper pitch Solas. I am active on a couple of the Donzi enthusiast websites, and am trying to find someone who might want to swap a 21 for my 19. It is WAY more efficient (2% vs 19.7% slip). It is odd though, because I used to race outboards in the late 70s/early 80s, and I realize that is old technology, but speed props were always smaller diameter with more pitch and rake, and tug boat props were always larger diameter and shallower in pitch. I'm having a hard time accepting that a large diameter, smaller pitch prop would be the way to go.

Maclin, I would love a duoprop lower, there are tons of positives, and virtually no negatives, but that is a pretty significant expense.

It is all moot anyway, because 1.61 are the tallest gears available (or at least listed) for the AQ290.

Thanks for the input.

 

[Tail_Gunner]

Re: Should I change ratios in my Volvo AQ290?

THis could be simple you have high slip with a 14" hub and low slip with a 15" hub. Question how well do the 14 hubs snug up to the drive...is there any size difference say 1/4 or 3/8 between the to?

 

[Reflexx175]

Re: Should I change ratios in my Volvo AQ290?

Both hubs fit EXACTLY the same. I'm sure it has to do with the increased efficiency of new prop design.

 

[The X man]

Re: Should I change ratios in my Volvo AQ290?

I have tried a Solas 19, a Solas 21, a Michigan 21 and an Ultra 24. I will admit there wasn't alot of difference with the two solas props, just about 200 rpms. I bought a 21 and am generally happy with it. I have a stock 260 hp motor and spin the solas 21 to 4,800. The ultra also spins to 4800. I also have a stainless Michigan 21. That one will spin to over 5,000 rpms. I don't know what it is, those old ultra props run very well on old Donzi's with volvo drives. While it has high slip #'s, it handles like a dream. The Michigan isn't bad either. I tend to get a bit of stern lift at times from the Solas props, maybe because of the larger diameter. Dealing with propping the older volvo drives is tough. There must be someone near you that you could borrow a solas 21 or 23 from. The ultra I tried was borrowed. I'd love to get my hands on an ultra 24 for myself.

 

[Reflexx175]

Re: Should I change ratios in my Volvo AQ290?

I have tried a Solas 21, and it is about 200 rpms less, and about 3 mph faster. I lucked out and bought an Ultra 24 off eBay. It is in great shape. It is the fastest of the 3 props, but you have to trim it like an outboard with the bow high. The Solas props carry the boat flatter, and giving them more trim does not seem to do anything for speed or bow lift. I have to find a set of gears for my eDrive.