Just bought a 1988 Catalina 30. The boat has an M25XP engine. The prop is a 3 blade 10x13. I've only had a few weekends on the boat but find when motoring we're not hitting the speed the boat is capable of. At 2800-2900 rpm I'm seeing a speed of approximately 5.6 knots. I'm of the opinion the prop needs to be repitched. I'm looking for recommendations. Thanks.
- Status
13x12 should work
The standard 2 blade is 13"(dia comes first)x12. Thats under pitched, but it does little good to go higher, as there isn't enough blade area at 13". Thats is equivalent to your 13x10 3 blade, but because of the extra blade area the 3 blade offers, you probably can benefit from going to 13x12. While the rule of thumb about seeing if you can reach rated( in your case, 3200) rpm at full throttle is usful, it only can tell you if you are overpitched, as underpitched props will still stop at 3200 due to the governor. I have a 3-blade Max-prop at 13x12(26 degrees), and get 5.5 at 2700 cruise. (89 fin keel, same engine)
The standard 2 blade is 13"(dia comes first)x12. Thats under pitched, but it does little good to go higher, as there isn't enough blade area at 13". Thats is equivalent to your 13x10 3 blade, but because of the extra blade area the 3 blade offers, you probably can benefit from going to 13x12. While the rule of thumb about seeing if you can reach rated( in your case, 3200) rpm at full throttle is usful, it only can tell you if you are overpitched, as underpitched props will still stop at 3200 due to the governor. I have a 3-blade Max-prop at 13x12(26 degrees), and get 5.5 at 2700 cruise. (89 fin keel, same engine)
Proper pitch
Dear Dave: Thanks for your note. Before I would go out and have the current prop re-pitched or a new prop purchased, I would determine if whether the current prop is properly loading the engine. This can be done away from the dock motoring in flat calm waters. Once the engine is up to operating temp, give the engine full throttle and witnesses the achievable Rpm. If RPMs fall short of max rated amount you will need to reduce pitch which reduces engine load. If RPMs are greater then the max rated you will need in increase pitch to increase load to the engine. In doing this you will determine whether the engine is operating at optimum efficiency. If the RPMs are dead on to the max rated, increasing pitch to increase speed will harm your engine. This puts the engine in an overloaded state causing carbon and heating problems. Because the propeller is only capable to transmitting what torque is being produced by the engine, increasing pitch will not always increase the speed. When pitch is increased something will have to suffer for the increased load. Hope this helps. Sincerely, Kevin L. Woody
Dear Dave: Thanks for your note. Before I would go out and have the current prop re-pitched or a new prop purchased, I would determine if whether the current prop is properly loading the engine. This can be done away from the dock motoring in flat calm waters. Once the engine is up to operating temp, give the engine full throttle and witnesses the achievable Rpm. If RPMs fall short of max rated amount you will need to reduce pitch which reduces engine load. If RPMs are greater then the max rated you will need in increase pitch to increase load to the engine. In doing this you will determine whether the engine is operating at optimum efficiency. If the RPMs are dead on to the max rated, increasing pitch to increase speed will harm your engine. This puts the engine in an overloaded state causing carbon and heating problems. Because the propeller is only capable to transmitting what torque is being produced by the engine, increasing pitch will not always increase the speed. When pitch is increased something will have to suffer for the increased load. Hope this helps. Sincerely, Kevin L. Woody
Re: Proper Pitch
Kevin, Thanks for your response. With the current 10 x 13 prop I have no problem reaching max engine rpms. In fact the rpms come up so fast it's, as a buddy of mine said, as if there's no prop at the end of the shaft. There's no indication/sense/sounds etc., that indicate the engine is being loaded down by the prop biting into the water.
Kevin, Thanks for your response. With the current 10 x 13 prop I have no problem reaching max engine rpms. In fact the rpms come up so fast it's, as a buddy of mine said, as if there's no prop at the end of the shaft. There's no indication/sense/sounds etc., that indicate the engine is being loaded down by the prop biting into the water.
Very interesting!
I know this sounds wierd, but I have a Universal 5411 that produces 11 hp. I'm using the standard 2 blad prop that came with the boat. What I find interesting is that I can achieve the same speeds with my little engine that could.
I cruise at 5.5-5.7 and top out near 6.3 knots. Humm... OK, what's up with you all that are having problems with 2x as much power?
I know this sounds wierd, but I have a Universal 5411 that produces 11 hp. I'm using the standard 2 blad prop that came with the boat. What I find interesting is that I can achieve the same speeds with my little engine that could.
I cruise at 5.5-5.7 and top out near 6.3 knots. Humm... OK, what's up with you all that are having problems with 2x as much power?Check the transmission...
Check the transmission. My 87' 30 that I recently purchased was having the same symptoms, found out the transmission was slipping. Also make sure the shaft and prop is free of barnacles! Good luck!
Check the transmission. My 87' 30 that I recently purchased was having the same symptoms, found out the transmission was slipping. Also make sure the shaft and prop is free of barnacles! Good luck!
BLADE AREA!
Because a 13" 2 blade prop can't absorb more than about 15hp. If you swing a 3 blade then the extra hp become useful. Are you SURE about 6 kts? I could just approach 5.7 with the 23hp engine and original 13x12 x2 prop., under optimum conditions. Things are different with the 3blade maxprop though.
Because a 13" 2 blade prop can't absorb more than about 15hp. If you swing a 3 blade then the extra hp become useful. Are you SURE about 6 kts? I could just approach 5.7 with the 23hp engine and original 13x12 x2 prop., under optimum conditions. Things are different with the 3blade maxprop though.
- Status