Art,
You said- "Our 34’ classic 1977 Tollycraft tri cabin has good condition twin 255 hp gas engines, 1.5 reduction, and straight shaft... it twists well balanced and clean 17 x 16 three blade props (I call them my “little cutters”). As a clean bottom, hard chine planing hull with 33” draft, 11 degree deadrise, and loaded weight of 21,000 lbs she performs well in exact engine synchronization with no vibration at hull speed (7 to 7.58 knots), planing cruse speed (16 to 17 knots), and WOT (21 to 22 knots). These props were mathematically designed by Tollycraft Corporation engineers in coordination with outsource engineers to match this boat’s range of requirements. I am completely satisfied with performance rendered."
I am purchasing a 1977 Tollycraft 34 Tri-cabin with twin 255 hp gas engines running well. The current owner reports a maximum speed of 16-17 knots WOT. I am wondering what gas consumption you have and what RPMs you find at what speeds... If you can post those results I would be very appreciative. Greg
Wow Greg - You pulled back a few years to get my post; i.e. 2 days after Christmas in 2012... good going!!
So... I want to be as clear as possible regarding twin engine RPM's [at different throttle settings], prop size, nmph speed and nmpg fuel usage.
RPM: I watch each engine's RPM on old-school tachometers [seeming to be the 1977 originals still working though!]. Between the four tachs [two in salon and two on bridge] there seems to be a not too accurate RPM correlation between the lot that varies from 100 to 300 RPM differences at different throttle settings. As well, I always set both engines cooperative RPM [at what ever throttle setting I'm running] on a light sensor synchronizer... as well as my ear synchronizer, which due to decades experience I feel is quite accurate. When the engines are synced into same RPM [singing a pulsating mechanical melody with one another] I feel they are then darn close to exact turns per minute compared to one another [give or take 25 to 75 RPM] Having said that - The tachs can still vary their RPM reading from 100 to 300 RPM difference; and I don't care! It's how good the well running engines feel while performing in synchronization that interests me most. Also, I do not push engines past their own running-comfort level... except... for momentary needs of testing purposes or a quickly needed must-do to get the boat away from immediate danger.
Prop Size: Prop calculations are one of the most vexing math-design problems ever encountered in power boat use as they need shape, weight, size, # of blades and shaft diameter comparisons/configurations that cooperate with a boat's hull shape, size, draft, keel, and rudder as well as a myriad of other complications regarding prop match to engine size/type/HP and reduction gear numbers utilized. In other words... IMHO... only experienced propeller engineers can come close to "THE" Correct Prop! Therefore, due to millions spent and years/decades of tests performed by good builders of boats [such as Tollycraft]... I follow the doctors orders and use the prop they recommend.
Here's a bit of info I've gleaned regarding props for 34' Tollycraft sedans and tri cabins. As you can see, there are more than one chef in the kitchen
"Mid-70's 34 Tollycraft, with 255 HP gasoline engines came with 17" x 15" props. Most latter 70’s 34 tri cabins came with props around 17x16 to 17x17. If you reduce the pitch down to 12 you will find that this will put you in the sweet spot of the performance curves. With this HP, gear reduction and 9k+ lbs of engines and trany. Any more pitch will result in poor mid range and top end performance.
1983 34 tri cabin with 270 Crusaders was originally propped 17X16. First owner reduced the pitch to 14 which let engines run free but required more RPM. We repitched to 17X15 which is much more responsive, especially while docking. We run displacement most of the time, 11 knots GPS at 22-2300 RPM, about 9 gals per hour. Will top out at 4200 WOT
Anyway - Although I've never needed our props tuned I am going by the 17 x 16 numbers the original owner told me are on our baby. His decades experienced marine technician that had cared for the Tolly for 10 years agreed.
Now, if you are still reading and have not fallen asleep... I'll answer your question and make a bit of comment. See
bold inserts in your quote below.
"I am purchasing a 1977 Tollycraft 34 Tri-cabin with twin 255 hp gas engines running well. The current owner reports a maximum speed of 16-17 knots WOT." -
Seems pretty slow to me. How many hours on engines - you sure they are running at full power output? [B]Was bottom perfectly clean with new paint? What size props? Were all tanks full, 1/2 full or empty? Was boat loaded. partially loaded or un loaded of food, cloths, tools, spare parts? How many persons aboard? [/B]
"I am wondering what gas consumption you have and what RPMs you find at what speeds... If you can post those results I would be very appreciative."
1. Running on one engine at 4 to 4.5 knots we get right at 3 nmpg
2. Running on twins at 6.5 to 7 knots it's 2 nmpg [7.58 knots is calced hull speed]
3. Running on full plane at 16 to 17 knots she averages 1 nmpg
4. WOT at 21 to 22 knots I imgine [maybe] 1/2 nmpg. Never kept her at that speed but for a minute or two.
Speeds were taken at slack tide by GPS. Fuel usage was calced by full tanks that were refilled at end of run time... with appropriate math used to enter distances traveled in regard to gallons of fuel used.
Cheers and Good Luck!
Art