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Ernie
08-25-2009, 04:38 PM
My name is Ernie and I'm a new member. Also new to Donzi ownership. I've just bought a 1994 Classic 18 with a Volvo 5.8 FI 300hp engine and Volvo SX outdrive. It has a 21 pitch aluminum prop that turned 4900 rpm's and 59 mph (gps) with a slight bend near the tip of one of the blades. I think I can probably go to a 23 pitch maybe more. I would like to get near 65 if I can. I'll go either aluminum or stainless whatever works best. Problem is I can't seem to find very many props for that outdrive. Any ideas?

MOP
08-25-2009, 04:50 PM
Welcome! If it is a 300 it is shy on speed, but putting bigger prop may just slow it even more. I would get what you have fixed then run it and post the numbers, the selection of props for the drive is on the slim side. You may need to have the one you have tweaked, the 300=4900=59 has me thinking you may have other tuning issues. Work with what you have and keep us posted there is a really good prop guy up here but he needs proper feed back.

Phil

blackhawk
08-25-2009, 06:04 PM
My 1996 "fake Donzi" ran the best with a 23 Viper prop. I tried 5 or 6 props(Turbo, Mirage+, etc) and came back to the original Viper and it ran 66-68mph at 4900-5000rpms.

Different hulls but the Viper is a great performing prop.

Morgan's Cloud
08-26-2009, 06:37 AM
I think Phil is right ...

I test ran an 18 with that same engine at the MIBS in '93 and it was a lot faster than 59 !

Once you know that everything is in spec with the engine BigGrizz can set you up with props. He has a lot of stuff that adapts to the Volvo units as I found out when I was trying so sort out my handling issues with my SX-A .

Steve

MOP
08-26-2009, 07:00 AM
The prior owner may have down propped it not knowing what was causing the loss of power, you are missing by "Guess" 70-80HP that is somewhere in the ballpark in relation to the MPH drop. The steps I would take are the basics, always first pull the plugs keep them in sequence when you remove them and give them a careful looking over. Next a compression check if the turns up poor then no need to go any further. If you don't know engines or are unsure what you are looking at post pictures and compression readings. Something is not right can very well be something simple but never know you are down quite a bit in power! I would do the above before you run it anymore, you just never know what might be going on inside and engine!

blackhawk
08-26-2009, 07:57 AM
Phil, what about the fact the prop is aluminum? I have limited experience with aluminum props but I have always seen a significant speed loss going from ss to aluminum. Plus his prop is damaged. Just throwing it out there.

RedDog
08-26-2009, 07:58 AM
The bend in the prop tip throws all speculation out. Fix it first - cheap fix - and then report GPS speeds along with RPMs.

Of find a 21 / 23 pitch prop in good condition and report back.

Stainless is the way to go

Lenny
08-26-2009, 09:00 AM
A couple of things. First, what is the gear ratio in the SX drive ? 1:43, 1:51, 1:60, ???

Your theoretical top speed with a 21" prop, and 4900 rpms with a 1:1 drive would be 97.44 mph. Assuming you have a 1:51 drive ratio that puts you at 64.53 mph before any slip calculations. You have about 9% slip with that prop combo IF that is a 1:5 drive.

Get the slip down to about 5-6% ( as in fix the thng) and you will see 61-ish. Those are respectable numbers for those rpms. All but perfect ( if, again, IF ) that is a 1:51 drive.

What I would be hunting for (after fixing the prop) is the "missing" few hundred rpms that you need to get you to your desired number. 4900 seems low for that combo.

What is your altitude where you boat?

Is it injected ?

Is the hull fully trimmed out at WOT and dragging NO tab ?

What is the drive ratio?

Here is a start... :D


The Volvo SX is an exceptionally robust drive, available in 8 different ratios (1.43, 1.51, 1.60, 1.66, 1.79, 1.89, 1.97, 2.18) Both the upper and lower units are reduction gearsets- the upper is offered with two differing ratios, the lower in five ratios.

Welcome aboard :)

BigGrizzly
08-26-2009, 01:10 PM
This is 300 guys not any more. 65 is a stretch. My comment on the viper is that if it works for you use it, however I have never found it to be the best with handling and speed. On my own boats I have several different pitches of the same prop that yield different things. In the Criterion I use a 28, 29 and a 30 pitch depending on conditions and what I want to do.

blackhawk
08-26-2009, 02:28 PM
I was merely stating the Viper worked on my boat and worked well. My hull wasn't a Classic 18 but similar. 19', 7' beam, 24* deadrise, 2800 lbs with a Volvo SX pretty deep.

Ernie, I would borrow or steal a SS 21 or 23 prop in good condition and get a baseline.

BTW the SX drive is a GREAT drive!

MOP
08-26-2009, 03:31 PM
My initial post basically says fix what you have and let us know, IMO 300 in an 18 should get into the 60's. My 22 with the Tempest 300 did 61.3, his is a lot less boat to push.
As far as the aluminum "vs" S/S when I took my 16 up to the 05 1K run I put an aluminum wheel on same pitch and noticed little to no change, to be sure nothing significant!

Phil

Ernie
08-26-2009, 09:07 PM
I thank everyone for their ideas and I did change a few things with satisfactory results. It now turns 4800 rpms, down a little, but now runs 63 (gps). I think that this all I can really expect with only 300hp in a small block ford. The boat came with one of those trim deals on the outdrive. I took it off and lost about 200 rpm's but picked up speed to 63mph on the gps. I believe the trim thing was causing some turbulance around the prop and some drag. Now I will fix the prop and maybe try a stainless, but my past experience with small hp engines is that the gain from aluminum to stainless is very small if any at all. My biggest problem now will be to find someone with the diagnostic toll for that fuel injected 5.8. Apparently you have to have one to diagnose any problems if I have any. Unless, someone knows a short cut. Thanks again, Ernie

Lenny
08-26-2009, 11:22 PM
Great post :yes: What is your drive ratio?

You are basically close to where you should be STOCK.

This is GPS keep in mind, NOT, dash speedo.

Thanks for play'n :D

Welcome once again, :yes:

Morgan's Cloud
08-27-2009, 06:50 AM
Out of curiousity , what exactly is the 'trim thingy' that you took off ... was it one of those 'Doelfin' things ?

BTW , Lenny makes a very valid and often overlooked point when he brought up the issue of 'altitude'.


Often members here give no 'location' in their sign up info and it would be easy to understand that one would automatically think in terms of 'sea level' when doling out free advice ...... :kingme:

MOP
08-27-2009, 07:25 AM
I am going to guess the trim thing was a Dolfin Stabilizer or similar, the prior owner of my 16 put one on, it turned out to be a joke. Yes it did plane off much faster but when you trimmed up for speed the the stern started do dive and slowed the boat down a bunch.
On the prop thinking yes stainless will give a little more speed but at what cost when you hit something, I know from many years of experience that aluminum is a safety net. Hit something like a log with stainless and the cost of repairs will sky rocket, some things may not show up right away like in Todds case and many others. With aluminum the prop destructs on impact transmitting far less to the drives internals! For normal cruising in iffy waters aluminum is a good choice, that last MPH is far less important then your drive.

Ernie
08-27-2009, 05:07 PM
I'm pretty sure the drive ratio is 1.51 to 1. The trim thing, I believe, was a Doel Fin. I've seen them in Bass Pro shops. They are a huge red, silver and black. I'm located in San Antonio, Tx. No altitude problem but, we are in day 63 of 100 degree plus days. Very high humidity. I remember my days of horsepower, that they run much better in cooler weather. I have seen as much as a 3mph gain when there was a 20 degree change. Having grown up in Anchorage, Alaska, I normally wouldn't complain about the heat, but enough is enough. I don't even want to take the boat out until 8pm. Anyway, I think I am good to go with the performance the way it is. Besides, no matter how fast you are, there's always somebody faster and richer.

Lenny
08-27-2009, 11:40 PM
no matter how fast you are, there's always somebody faster and richer.

BINGO :D

San Antonio/Anchorage,... talk about polar opposite environments :yes:

MOP
08-28-2009, 06:50 AM
With all this being said now go and enjoy your DONZI!

Phil

thescooter
09-01-2009, 02:22 PM
well its like this.
it is cheaper to keeper than eat a new one.
i mean a aluminum prop over a Stainless.
for the few 1 to 2 mph and you hit something it is cheaper to throw alway a aluminum prop than a stainless prop and possible damage a drive.
is there other problems that you are not telling us about.
does the boat get out of the water from the hole and from a stand still?
does the boat handle correctly when you slow down in a followwing sea?
does the prop have any cavitation burns or markings?
if you do not have any of the above issues stay with what you have.
i do not think switching to SS will increase your speed enough to warrant a investment or possible major loss to your drive.
source:
been there do that , fixed that and repalced that for 2mph and hit floating junk in the water.

BigGrizzly
09-01-2009, 05:34 PM
Not to change your mind because you hit things BUT the SS swtyles are faster top and ont of the hole they handle better, if you get the right one and they get better economy. If you are prone to hit things the aluminum is cheaper and easier to fix or replace, especially rocks. if it is branches the ss is better be4cause it does not take much to bend or nick.

Ernie
09-01-2009, 08:35 PM
The boat came with a Mich wheel that is 14 1/2 by 19 pitch and had a spun hub. I sent the stainless to Nettles Props in Austin, they said they could repitch it to a 21. I assume it stays a 19 at the base and then works to a 20 midway and then ends up a 21 at the tip. If so, it might make for a good prop for this set-up.

Ernie
09-03-2009, 09:25 PM
I got back my stainless mich wheel. It was a 14 1/2 x 19 and I had a new hub put in and repitched to 21. It was better out of the hole than the solas alum. but top end stayed the same except the stainless turned 200 rpm's more than the 21 solas. I assume because it started out as a 19. Plus, I've never read where anyone said go to a mich wheel for performance. I guess I need to find someone with a 21 and or 23 stainless performance oriented prop for my Volvo SX outdrive. I believe they are the same as a King Cobra (19 spline).