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HELP! Correct Prop size?

Yobbo

New member
Hello, I have just joined and am looking for some help to find the correct prop size for my boat

My boat is an Australian (I am in Australia) 1989 Riviera 30 Fly bridge, the single engine is a AQAD40B and the Stern drive is a DP280.
I purchased the boat about 9 months ago and was told at the time it is over propped, I can only reach 3200 RPM WOT at about 10 knots. I have been working away fixing and upgrading a long list of jobs to do on the boat and have now come to getting the right sized props, I have been told I need 2 sizes smaller to be able to reach the 3600 RPM WOT. I have removed the rear 4 bladed prop to try and find a part number, which would give me a starting point to ordering new props 2 sizes smaller. However the only number I can find is the number 19 stamped into the hub, I have taken the whole hub back to alloy using paint stripper to remove years of anti foul and a wire bush on a grinder and the number 19 is the only number I can find. I know I need to order A series props to suit the DP280 stern drive. Would anybody have any suggestions on where to start? the boat weighs about 5000kg ( 11,000 lbs), engine and stern drive are mid 80`s both in good working order. I know the boat is under powered and is never going to go anywhere fast, but I would like a suggestion on what A series props to use, thank you for your help
 
UPDATE: After many phone calls, emails. I have been told that my current props that show the number 19 are aftermarket and equates to a A4 set of duo props, I have also been advised to purchase A3 props, which I have done and I will fit later this week, I will update again on the outcome, I hope this helps someone in the future
 
Hi Yobbo, I must check with you, something is very strange here, You hava kit of A4 on your stern drive today, do you know if the previous owner have had this props since the boat was new?, do you know anything about speed of the boat then?
If you have a stern drive geared for a diesel engine, you should get about 28 knots at 3600RPM with a pair of A4 props, If your boat was over propped and will only reach 10kn, your engine would not reach 3200prm, I would say it would reach max 2500rpm, 3200 RPM is your crusing RPM on this engine.
If your engine are fresh and healthy, you should reach close to 20kn on full trottle( I assume the engine is a 165hp).
I think you have some other problems with your installation, you should check the gear ration on your stern, check the hubs in the props, I am little interested what will happen with the boat speed and engine RPM if one of the hubs are spinning, I do not have any experience of DP props that have hub problems, just SP.
If you just change to A3 props, the engine RPM will increase about 300RPM, but the speed will be the same.
I am in Sweden, Still 50cm of rock solid ice.
 
I think Martin is making sense here. You probably need to get someone with the Volvo prop calculator to run your boat weight, hull type, gear ratio, and prop size through and tell you what speeds you should get. At the moment I think Martin is suggesting that you have massive prop slip happening, and that you need to find out why. I haven't got a calculator for the Volvo props, but I suspect Martin is correct.
 
UPDATE: I put the new aftermarket A3 Props on and I can now reach 14knots at 3200rpm, I can not get any more RPM than 3200, could it be that the engine is governed to 3200 rpm? I am unsure of the engines past, the boat has had a few owners before me and all of them sold the boat as it was to slow for them. I have spoke with other Riviera 30 owners with the same engine 165 hp AQAD40B with 900 hours and DP280 stern drive combination and all have complained that there boats are underpowered. To be honest I am quite happy with it, I knew the boat was slow when I brought it but it is quite economical, I now cruise at around 10 knots at 2800 rpm and the engine does not sound like it is working to hard, it`s a old engine but I would like to get a few more years out of it yet. I have no plans to repower it as I would like to buy a bigger boat in a few years. Thank you
 
Yobbo - I think you are fighting the laws of nature here. At 14knts your hull is probably not properly on the plane, and you don't have enough hp to get it there. If you could break it free, your engine would probably spin up a bit more and keep it going faster, but first you need to break it free. Have you tried going out on a day with some good swells to see if you can surf it up to maybe 16knts or so where it probably is planing properly? You could also try using the wake of a faster boat to get you 'surfing'. With the boat transitioning from displacement mode (say up to ~8knts) to planing mode it required lots of hp to break the hull out onto the plane. It looks like your engine just won't do it, which wouldn't be a real surprise given how inefficient those old Riv hulls mostly were. By the way, your economy at 10knts on a 30ft planing hull will be horrible compared to what you can achieve at 7.5 - 8knts. At 10knts you are definitely riding the transition hump and that is a big hp/fuel sucker.
 
Aliboy, Thanks for your reply, I feel I am asking a lot from a old small engine on a large boat. Over Easter with the old A4 props we cruised at around 7 knots and covered approx 140km / 85 miles and used 105L / 27 USG or around 10L/ 2.6 USG per hour, our trip actually cost more in beer than it did diesel, at 10 knots my consumption was around 17.5 L /4.6 USG per hour. Now I have the new A3 props fitted and have had the boat out only twice since fitting them I have noticed at 12-13 knots I am using around 25L / 6.6 USG per hour. My next trip out I will sit at 10 knots there and 8 knots back and record the fuel usage.
 
Fuel numbers look a little high (rough estimate). My twin 330's pushing a 43ft game boat are only 15lph at 8knts, and from memory around 10 - 12lph at 7knts. If your 25lph is at ~3000rpm or so you seem to be a little high there as well, but that is probably due to the hull sitting on the transition 'hump'. May be an indication though that you don't want to be constantly at that speed if it is in fact overfuelling. Any black smoke visible?
 
You should reach 3600RPM at WOT. 3200 is to low, your engine is struggeling alot, If you have the possibillity to test A2 propps, I would have done this, This will teoretically increase the RPM with 300RPM, An i think your top speed of the boat will increase, your fuel consumption will decrease.
About fuel economy, some of my own experiense, not perfect but It is usefull.
A modern Diesel engine gives you app. 3,5hp/hour/liter of fuel, so if you have a diesel engine with 200hp, it will consume 57 liter/hour on full trottle, and att crusing speed of 75% of max speed, it will consume 70% of full trottle consumption, are you with me?
So if you have a Fairline 48 with twin Volvo 74(480HP each), and top speed 30 knots, the fuel economy will look like this:
480x2 = 960hp, = 274 l/hour = 9,1L/nm at full trottle = 6,4 Liter/nm at crusing speed.
A gas engine gives you 2,5hp/hour/liter of fuel.
Test on your own boat, as see if this calculation gives you close figures.
And If you go below plane, the consumption is much lower than this.
 
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