Prop size
Prop size
We are new this year and have purchased a alwest 370 with twin 5L Mercruser 898 legs (1977-82). We hav not had the boat over 6 Knots (1500 RPM). We bought her in the water not running so when I got her running I did not want push my luck because of poor bruised and battered props (15X19 3 blade) amd unkown sterndrive condition. A little grease and oil and all should be well this year. I would like to see normal speeds for this boat and did not know it had a planning hull when we bought her (Georgina Marie her new name). Can anyone help?
Re: Prop size
Hello Ellen,
Welcome ot the world of Alcans & Alwests !
In the "Technical" section of this site, see the first series of info under the title Hull Dynamics.
This data was created/provided by the site webmaster (who also has an Alwest)
His newer 4.3L V6 engines are likely very close to the HP produced by your older 5.0L V8s
This would be a very good place to start re prop size and speed potential.
Rob
Welcome ot the world of Alcans & Alwests !
In the "Technical" section of this site, see the first series of info under the title Hull Dynamics.
This data was created/provided by the site webmaster (who also has an Alwest)
His newer 4.3L V6 engines are likely very close to the HP produced by your older 5.0L V8s
This would be a very good place to start re prop size and speed potential.
Rob
Re: Prop size
Hi Ellen
I will take a stab at the answer. Your boat should be able to do somewhere around 30MPH with the power system you describe. To the best of my knowledge, the 5.0l engines are around 200HP. This is more than the original 170HP 6-cyl that probably came with the boat. However, the V-8's are quite a bit heavier than the I-6's. Too much weight and the boat will not be able to plane. The engines alone would not be enough added weight to prevent planing. However, the typical 30+ years of modifications and accessories can add quite a bit of weight. I probably pulled 50lb of unnecessary electrical wire off of Buster when I first got her.
Without knowing the gearing in the legs, it would be hard to give good prop advice. Even if we knew the gearing it would still be hard to give good prop advice. The 15" diameter is probably good. Big enough for good thrust and not so big as to be hard to turn. The pitch may or may not be right. Start with the assumption that the pitch is right and do some speed trials. Assuming the boat gets up and planes, you are looking to ensure that the engine RPM at wide-open throttle is within specification for the engines. Too high - more pitch. Too low - less pitch.
Cheers
David
I will take a stab at the answer. Your boat should be able to do somewhere around 30MPH with the power system you describe. To the best of my knowledge, the 5.0l engines are around 200HP. This is more than the original 170HP 6-cyl that probably came with the boat. However, the V-8's are quite a bit heavier than the I-6's. Too much weight and the boat will not be able to plane. The engines alone would not be enough added weight to prevent planing. However, the typical 30+ years of modifications and accessories can add quite a bit of weight. I probably pulled 50lb of unnecessary electrical wire off of Buster when I first got her.
Without knowing the gearing in the legs, it would be hard to give good prop advice. Even if we knew the gearing it would still be hard to give good prop advice. The 15" diameter is probably good. Big enough for good thrust and not so big as to be hard to turn. The pitch may or may not be right. Start with the assumption that the pitch is right and do some speed trials. Assuming the boat gets up and planes, you are looking to ensure that the engine RPM at wide-open throttle is within specification for the engines. Too high - more pitch. Too low - less pitch.
Cheers
David
Re: Prop size
Horsepower 220 Hp and some dealers (not really nkowing the boat) have recomended 16X14 another 14 3/4 X15 the boat came with 15 1/2X19 and topped out at 1800 to 2000 RPM how ever drives were not maintaned and I got it in the water so caution was in the air. The drive ratio is 1:1.5
Re: Prop size
You say the boat "topped out" at 1800 to 2000 RPM. Was that at Wide-open throttle (WOT)? If so, then there is something really wrong. If by being cautious you mean you limited the throttle, then I am not sure we can do much with the data point.
I wish I could remember what gearing I have on Buster. It is either 1.79:1 or 1.89:1. I can tell you I am running 15 x 22 four-bladed stainless steel props. I get 4200RPM at WOT with the legs down at 36MPH. When I trim the legs up a bit to get the last 2MPH the RPMS come up to around 4500 which is about right for the engines (max RPM = 4800).
at 4400 RPM, at 1.79:1 I get a pitch speed of 51MPH. If the ratio is 1.89:1, the pitch speed is 48MPH. Assuming you have a similar horsepower, you will want a similar pitch speed. However, your gear ratio is different, and my recollection is that the 5l V-8 should be limited to 4000RPM. So... at 4000RPM and a 1.5:1 gear ratio, and a 19" pitch, you get a pitch speed of 48MPH. I suspect the 19" prop is about right. However, there are all kinds of factors that will prove me wrong.
I wish I could remember what gearing I have on Buster. It is either 1.79:1 or 1.89:1. I can tell you I am running 15 x 22 four-bladed stainless steel props. I get 4200RPM at WOT with the legs down at 36MPH. When I trim the legs up a bit to get the last 2MPH the RPMS come up to around 4500 which is about right for the engines (max RPM = 4800).
at 4400 RPM, at 1.79:1 I get a pitch speed of 51MPH. If the ratio is 1.89:1, the pitch speed is 48MPH. Assuming you have a similar horsepower, you will want a similar pitch speed. However, your gear ratio is different, and my recollection is that the 5l V-8 should be limited to 4000RPM. So... at 4000RPM and a 1.5:1 gear ratio, and a 19" pitch, you get a pitch speed of 48MPH. I suspect the 19" prop is about right. However, there are all kinds of factors that will prove me wrong.
Re: Prop size
Thanks for all the help I think I got the best shot at normal with all the maintanence caught up hope to see everyone one on the water next year.