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Author Topic:   1978 13-footer with 20-HP Four-cycle

tjxtreme posted 09-28-2011 10:08 PM ET (US)
I finally got around to clocking my ride on GPS last night. It's 1978 13-foot hull, lightly loaded, and has a 2007 Yamaha 20-HP four-cycle outboard engine with CMC PT-35 trim unit. [Top speed was] 28.5 MPH. This is comparable to other threads I have read here. The propeller is a Solas 9.25 x 10, but I know nothing about propellers--this is just the one that came on it.
Sojo81007 posted 10-06-2011 07:51 PM ET (US)
That really surprises me! I'm going to have to take a GPS and see what my 1968 is doing. With the 18hp Fastwin it feels like it's going about 15mph and with the 40hp Big Twin it feels like it's doing about 30mph. I've got to check this out.
dg22 posted 10-06-2011 11:28 PM ET (US)
I was surprised when my brother clocked me at only 33-MPH with a 50-HP on my 1967 13-footer--no power trim or jack plate. I think with the hooked hull design, if you don't have power trim to get some bow lift you're not going to gain much with the extra hp. Having said that, I can see how tjxtreme is getting 28-MPH with the CMC trim unit with only a 20-HP.
leadsled posted 10-07-2011 08:02 AM ET (US)
[A speed of] 28-MPH is great with a 20-HP. My Yamaha and Tohatsu 25-HP two-cycle outboard engines both ran my [1967] STANDARD to 24-nautical-miles-per-hour or 27.6-MPH. Back in the 1970's I had two different Mercury 50-HP outboard engines on my 13-footer and they were good for 38 to 40-MPH, and it felt like I was doing 50.
captbone posted 10-07-2011 08:50 AM ET (US)
The newer hulls may not have as much hook in them. My 13-foot Boston Whaler boat starts to bury the bow when you keep adding power. I suspect the smirked hull does not do this as much.
jimh posted 10-07-2011 09:20 AM ET (US)
How did one measure boat speed with accuracy in the 1970's?
leadsled posted 10-07-2011 10:21 AM ET (US)
Faith in ones speedometer? Actually a good question. I would say the ones with the hull mounted water pick-up back in the 70's should have been very close at speeds of 20-40 mph. The mercury propeller charts back then would give you a speed range for the given propeller's pitch. The readings I got seem to be right considering the different props I ran. If I remember right I had tried 15",17" and 19" props on my 1976 merc, The 17" prop ran the best at around 38-39.The 19 was a dog and was not worth the extra mile an hour. Briefly had a 2 bladed bronze racing prop that would break the 40 mph mark but was not a good all around prop and was stolen off my engine. When I bought my 19' banana outrage I bought a tournament waterskiing speedometer which was in kilometers per hour. It was a good size gauge maxing out at 80 kph (50 mph). The 175 Black Max Merc could get the dial to 80 kph. The tower of power 115 merc without power trim was good for 38 mph. Those readings seem to go along with mercury's propeller chart. Of course your right that their was no way to be sure how accurate the old speedometers were but I felt mine were close. And now with the GPS you have to take into account current with you and against unlike the speedometers which measure speed over water.
conch posted 10-07-2011 12:02 PM ET (US)
I used a stopwatch and ran over a measured and marked course.
Chuck
tjxtreme posted 10-07-2011 03:23 PM ET (US)
I'm happy with this performance when cruising with a light load, I think the CMC trim (and added setback that comes with it) really optimizes the performance of a fairly small engine. The fuel economy is excellent.
Based on what I have read, my propeller is also better suited for top end performance rather than hole shot... can someone verify this? How much difference might I see with a different propeller reaching plane under heavier loads?
Sojo81007 posted 10-07-2011 03:52 PM ET (US)
I think my 11.5" PJ31 is too small for my boat. I'm looking at getting a PJ35 (13.5") - do you think that would do much to increase speed?
Tohsgib posted 10-07-2011 09:00 PM ET (US)
My 40 does about 32 w/o PTnT and 38 with. I can see a 20hp setup correctly doing 28.
Binkster posted 10-11-2011 10:12 PM ET (US)
My 25 4 stroke merc/tohatsu will do 29mph on my '61 13 footer. Motor is on a 6" setback bracket and raised high with adjustable bracket. this is my best speed, what difference does it make, its an anemic speed, and its not a speed boat.
tjxtreme posted 10-11-2011 11:10 PM ET (US)
I'm not in it for the speed... nobody with a 20 HP is in it for the speed. However, this is one of the few ways of quantifying how well a certain motor/mounting/weight configuration works on a boat.

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Author	Topic: 1978 13-footer with 20-HP Four-cycle
tjxtreme	posted 09-28-2011 10:08 PM ET (US) I finally got around to clocking my ride on GPS last night. It's 1978 13-foot hull, lightly loaded, and has a 2007 Yamaha 20-HP four-cycle outboard engine with CMC PT-35 trim unit. [Top speed was] 28.5 MPH. This is comparable to other threads I have read here. The propeller is a Solas 9.25 x 10, but I know nothing about propellers--this is just the one that came on it.
Sojo81007	posted 10-06-2011 07:51 PM ET (US) That really surprises me! I'm going to have to take a GPS and see what my 1968 is doing. With the 18hp Fastwin it feels like it's going about 15mph and with the 40hp Big Twin it feels like it's doing about 30mph. I've got to check this out.
dg22	posted 10-06-2011 11:28 PM ET (US) I was surprised when my brother clocked me at only 33-MPH with a 50-HP on my 1967 13-footer--no power trim or jack plate. I think with the hooked hull design, if you don't have power trim to get some bow lift you're not going to gain much with the extra hp. Having said that, I can see how tjxtreme is getting 28-MPH with the CMC trim unit with only a 20-HP.
leadsled	posted 10-07-2011 08:02 AM ET (US) [A speed of] 28-MPH is great with a 20-HP. My Yamaha and Tohatsu 25-HP two-cycle outboard engines both ran my [1967] STANDARD to 24-nautical-miles-per-hour or 27.6-MPH. Back in the 1970's I had two different Mercury 50-HP outboard engines on my 13-footer and they were good for 38 to 40-MPH, and it felt like I was doing 50.
captbone	posted 10-07-2011 08:50 AM ET (US) The newer hulls may not have as much hook in them. My 13-foot Boston Whaler boat starts to bury the bow when you keep adding power. I suspect the smirked hull does not do this as much.
jimh	posted 10-07-2011 09:20 AM ET (US) How did one measure boat speed with accuracy in the 1970's?
leadsled	posted 10-07-2011 10:21 AM ET (US) Faith in ones speedometer? Actually a good question. I would say the ones with the hull mounted water pick-up back in the 70's should have been very close at speeds of 20-40 mph. The mercury propeller charts back then would give you a speed range for the given propeller's pitch. The readings I got seem to be right considering the different props I ran. If I remember right I had tried 15",17" and 19" props on my 1976 merc, The 17" prop ran the best at around 38-39.The 19 was a dog and was not worth the extra mile an hour. Briefly had a 2 bladed bronze racing prop that would break the 40 mph mark but was not a good all around prop and was stolen off my engine. When I bought my 19' banana outrage I bought a tournament waterskiing speedometer which was in kilometers per hour. It was a good size gauge maxing out at 80 kph (50 mph). The 175 Black Max Merc could get the dial to 80 kph. The tower of power 115 merc without power trim was good for 38 mph. Those readings seem to go along with mercury's propeller chart. Of course your right that their was no way to be sure how accurate the old speedometers were but I felt mine were close. And now with the GPS you have to take into account current with you and against unlike the speedometers which measure speed over water.
conch	posted 10-07-2011 12:02 PM ET (US) I used a stopwatch and ran over a measured and marked course. Chuck
tjxtreme	posted 10-07-2011 03:23 PM ET (US) I'm happy with this performance when cruising with a light load, I think the CMC trim (and added setback that comes with it) really optimizes the performance of a fairly small engine. The fuel economy is excellent. Based on what I have read, my propeller is also better suited for top end performance rather than hole shot... can someone verify this? How much difference might I see with a different propeller reaching plane under heavier loads?
Sojo81007	posted 10-07-2011 03:52 PM ET (US) I think my 11.5" PJ31 is too small for my boat. I'm looking at getting a PJ35 (13.5") - do you think that would do much to increase speed?
Tohsgib	posted 10-07-2011 09:00 PM ET (US) My 40 does about 32 w/o PTnT and 38 with. I can see a 20hp setup correctly doing 28.
Binkster	posted 10-11-2011 10:12 PM ET (US) My 25 4 stroke merc/tohatsu will do 29mph on my '61 13 footer. Motor is on a 6" setback bracket and raised high with adjustable bracket. this is my best speed, what difference does it make, its an anemic speed, and its not a speed boat.
tjxtreme	posted 10-11-2011 11:10 PM ET (US) I'm not in it for the speed... nobody with a 20 HP is in it for the speed. However, this is one of the few ways of quantifying how well a certain motor/mounting/weight configuration works on a boat.