Your thoughts on easy revving engine..

[Spi D]

The setup transfers power from the crankshaft via the flywheel and a fixed coupler to a shaft, two universal joints and fixed gears in the sterndrive. The sterndrive has an integrated cone clutch (in upper gear) that might need attention. The driveline ends in the prop shaft and unless the hub is slipping there is no concern at that point.

 

[wipe_out]

The setup transfers power from the crankshaft via the flywheel and a fixed coupler to a shaft, two universal joints and fixed gears in the sterndrive. The sterndrive has an integrated cone clutch (in upper gear) that might need attention. The driveline ends in the prop shaft and unless the hub is slipping there is no concern at that point.

So in other words I should be hoping to find a slipping prop because anything else is an out the water job?? :$

 

[Spi D]

I'd hope for a prop of too low pitch since this means no errors on the driveline

Is it very smooth when engaging gears at idle? The only task of the clutch is to ease engagement and should keep a firm grip apart from that. Drives like Merc Alpha use a dog clutch (like an outboard) and engages in a snap.

 

[wipe_out]

I'd hope for a prop of too low pitch since this means no errors on the driveline

Good point!!

Is it very smooth when engaging gears at idle? The only task of the clutch is to ease engagement and should keep a firm grip apart from that. Drives like Merc Alpha use a dog clutch (like an outboard) and engages in a snap.

From what I can tell it engages forward very easily.. Reverse is a little stiff but I had put that down to most likely being the cable..

 

[Scarron]

The valve train on those small blocks is pretty robust, you should be able to ease it up to the max revs gently on the throttle, being under load there is little danger you'll over rev it and you need to know exactly how fast it's going at max permitted revs to determine if it has a slipping hub or the wrong pitch. Don't forget to account for tide and let us know what numbers you achieve, someone can guide you from there once we know.

 

[Scarron]

So, go on then tell us how fast it went at max revs?

Then perhaps we can help with your dilemma..

 

[wipe_out]

So, go on then tell us how fast it went at max revs?

Then perhaps we can help with your dilemma..

I wish I could tell you..
I haven't had a chance to run her flat out yet, the days I have been down to the marina have been too rough to do the WOT test.. Also looking at the metoffice site the weather is pretty **** right through the weekend..

 

[Spi D]

Early mornings or evenings around 8.30pm should be most calm, in theory. Only problem being that climate doesn't give a **** about theories

Do you have info on gear ratio and ready-to-go total weight? If the mentioned 5500 lbs is true, 315 hp would push a clean, flawless planing V-hull to some 37 knots (calculated). Knowing the max recommended rpm at WOT and the gear ratio one can calculate the pitch needed.

Mercruiser rate these GM based V8's to 4800 rpm (some models 5200 rpm) so if we assume gear ratio is the common 2.15, the propshaft will spin at 2232 rpm. For 2232 rpm to produce a 37 knot forward movement, the pitch needs to be 20.3" (not taking slip into account).

Depending on slip you'd go for next size above, hence 21 or 22" depending on brand and availability. Good prop shops can change pitch to fine tune it (even change the exisiting prop).

Only test will show if calculations keep. Weight is crucial so if the 5500 lbs is factory specs you need to add fuel, gear and crew etc.

PS! Not knowing how your engine is built you should stay close to recommended rpm range although the same base engine comes rated up to 6200 rpm in stock versions. Purpose built racing engines on the same base go way higher, but that isn't relevant for your marine engine.

PPS! 90% of a props' properties come from design, 10% from material. A well designed alloy prop will work for most (in general avoid plastic or composite). Stainless will add that little extra or solve special issues because they can be designed more extremely, but come at a price.

 

[wipe_out]

Early mornings or evenings around 8.30pm should be most calm, in theory. Only problem being that climate doesn't give a **** about theories

Don't we know it!!

Do you have info on gear ratio and ready-to-go total weight? If the mentioned 5500 lbs is true, 315 hp would push a clean, flawless planing V-hull to some 37 knots (calculated). Knowing the max recommended rpm at WOT and the gear ratio one can calculate the pitch needed.

Just a quick google for a "larson 254 cabrio weight" brings up a few that are for sale.. They generally seem to list "dry weight" at 5200lbs.. So maybe my 5500lbs estimate was a little low once fuel, water, gear and people are added.. Boat has a 60Gal fuel tank and a 10Gal water tank.. Would probably have 3-5 people on board on average.. So I think 6200-6500lbs is probably closer to "normal" ready-to-go weight..

Mercruiser rate these GM based V8's to 4800 rpm (some models 5200 rpm) so if we assume gear ratio is the common 2.15, the propshaft will spin at 2232 rpm. For 2232 rpm to produce a 37 knot forward movement, the pitch needs to be 20.3" (not taking slip into account).

The "repower" engine is a GM base V8 and the book that came with it says the carb version max rpm is 4400-4800 (the EFI version is 5000rpm)..

I have no idea what the gear ratio is.. The original engine was a Volvo Penta 5.7GS but I believe that comes with a number of gear ratio options..

Depending on slip you'd go for next size above, hence 21 or 22" depending on brand and availability. Good prop shops can change pitch to fine tune it (even change the exisiting prop).

How much slip is reasonable?

PPS! 90% of a props' properties come from design, 10% from material. A well designed alloy prop will work for most (in general avoid plastic or composite). Stainless will add that little extra or solve special issues because they can be designed more extremely, but come at a price.

I have been looking at the options and it seems there are the few options for props.. Some with specific hubs some without.. Some 4 blade options and from what I have read 4 blade seems to have a lot of advantages with a small reduction in top speed as the only disadvantage so another consideration maybe??

Had not planned on using stainless for two reasons.. First is the price, especially while getting it right as I may have to buy two props if I can't swap out.. Second is that the boat stays in the water and I don't fancy the potential corrosion issues of the anodes and casings with a stainless prop..

Any prop shop recommendations? Either online or near enough to Brighton??

Thanks a lot for all the input by the way.. I have learned a lot about it over the last couple of weeks..

 

[Spi D]

At 6500 lbs max on a good day is calculated to 33 knots.

Slip can be almost anything depending on prop design, trim, hull and a lotta other parameters. Reason for testing...

Props are available from internet shops incl. fleabay. Good shops will allow test/swap as long as prop is as new. Some use brand specific fixed hubs, other use universal prop hubs that are mated to your drive by a replaceable hub.

Never seen stainless cause corrosion issues. Due to price they are popular among thieves.

http://www.steeldevelopments.net/ are recommended by many, no personal experience though. Prop design, number of blades etc. will have to be discussed along testing. In general 3-blade are best compromise on fast(ish) boats but 4-blade provide better grip (if needed!). More blade will on the other hand take more power to turn (more blade area=greater resistance).

 

[Chris_d]

This all seems to be getting far to technical, most problems are usually very simple. At the risk of sounding like a t*** I'm going to ask the obvious question, are you absolutely sure the drive is fully down?

I've had dodgey trim pump valves in the past and have had to stand on the outdrive to get it down the last 10 degrees or so, if the engine has been replaced the trim sender may have been incorrectly repositioned, it might look fully down but that last 10degrees or so which is not obvious maybe why its possibly cavitating and feeling like a slipping clutch!!

 

[wipe_out]

This all seems to be getting far to technical, most problems are usually very simple. At the risk of sounding like a t*** I'm going to ask the obvious question, are you absolutely sure the drive is fully down?

I've had dodgey trim pump valves in the past and have had to stand on the outdrive to get it down the last 10 degrees or so, if the engine has been replaced the trim sender may have been incorrectly repositioned, it might look fully down but that last 10degrees or so which is not obvious maybe why its possibly cavitating and feeling like a slipping clutch!!

Yes, the trim rams are fully retracted (no chrome showing) when the trim is pulled all the way down.. Could have a closer look but I'm pretty sure its down..

 

[Spi D]

This all seems to be getting far to technical, most problems are usually very simple.

Indeed. But switching engine, adding 45 hp, on the same driveline calls for attention to tech details, IMHO. If the new engine worked perfectly on the same gear ratio and prop, one would have to admit that not all of the 315 horses were awake...

 

[Cookee]

http://www.steeldevelopments.net/ are recommended by many, no personal experience though. Prop design, number of blades etc. will have to be discussed along testing. In general 3-blade are best compromise on fast(ish) boats but 4-blade provide better grip (if needed!). More blade will on the other hand take more power to turn (more blade area=greater resistance).

I can recommend Steel Developments through much personal experience - and I'm a boat builder.

 

[wipe_out]

I can recommend Steel Developments through much personal experience - and I'm a boat builder.

I emailed them yesterday and got a quick reply which is always a good thing..

Unfortunately they have no "exchange" policy.. In preference I would like the reassurance that I can swap the prop if its not right but failing that they seem a good bet.. Once I have done a WOT run I should have a better idea what's needed anyway.. Definitely considering a 4 blade for more "bite" on the water with a single prop on this size boat..

 

[GrahamHR]

Yes, the trim rams are fully retracted (no chrome showing) when the trim is pulled all the way down.. Could have a closer look but I'm pretty sure its down..

My boat has the DPS drive; which has the same upper unit as the SX. When my drive is fully down/ the pistons fully retracted, there's about 2" or so of the rods still visible. It was the same on an earlier DPS I had.

It seems strange that the drive on your's seems to go further down than on mine ( I know there are limit kits available, but they are to limit the maximum tilt angle, not the fully down position as far as I am aware). Perhaps the trim cylinders on the DPS have spacers to limit how far down they can go.

 

[wipe_out]

My boat has the DPS drive; which has the same upper unit as the SX. When my drive is fully down/ the pistons fully retracted, there's about 2" or so of the rods still visible. It was the same on an earlier DPS I had.

It seems strange that the drive on your's seems to go further down than on mine ( I know there are limit kits available, but they are to limit the maximum tilt angle, not the fully down position as far as I am aware). Perhaps the trim cylinders on the DPS have spacers to limit how far down they can go.

It might be that they have been painted when it was out the water about a month ago so the part that you see on yours is now painted on mine.. I will have a closer look next time I am down..

 

[GrahamHR]

I think they must be painted; I was working on my boat earlier and had a look, the pistons/ rods could not retract much further into the cylinder, as the upper unit would come into contact to the end of the ram cylinder bodies..

 

[Spi D]

When checking drive movement up/down you'll get it through two stages: Trim and tilt

Trim is for adjusting the angle during run
Tilt is for beaching or trailering, highly inefficient as the thrust goes upwards and not meant for driving the boat (apart from idle in shallow areas)

A full movement

 

[GrahamHR]

When checking drive movement up/down you'll get it through two stages: Trim and tilt

Trim is for adjusting the angle during run
Tilt is for beaching or trailering, highly inefficient as the thrust goes upwards and not meant for driving the boat (apart from idle in shallow areas)

A full movement

On a Volvo SX/ DPS that's only applicable if there's an electronic trim limit control unit fitted. It was standard on early DPS units, but wasn't on later units. I gather it was very unreliable. Certainly not standard in 1999, maybe earlier as well. A tell tale is the 3 wire sender on the outdrive rather than the 2 wire one (unless someone has fitted it as a cheap Ebay replacement for a 2 wire one , which I did!) plus a control box and a switch near the helm.

The video shows a pre 1999 drive ( different trim cylinders to later ones) so it may well have the trim control system fitted.