Thanks for replies. Some thoughts and additional clarification.
Q1 extended version: The Mercury 25 FOURSTROKE is made by TOHATSU. I figured the best [way to get advice about a propeller] was to ask people who own that engine.
In my experience, calculating prop slip and load tends to work more poorly the smaller the hull and the stranger the bottom of the hull, e.g. a 13-footer.
My experience is that, on bigger boats with deep V-hull, the trim angles matters but not as much as on flat bottom boats. On flat-bottom boats the trim angle is critical, and [the effect of trim angle] shows up in the GPH and MPG. Outboard engines on deep V-hull boats don't need to be adjusted for trim, while [the engine trim on] flat bottom skiffs does need some tweaking, based on changing load.
And I am confused whether a classic 13-foot hull acts more like a V-hull or more like a flat bottom skiff.
My concern about tilt trim:
Q1a: when you add people or shift around people in a classic 13-foot Boston Whaler boat (as made in 1969 and 1974) with a 25-HP engine, do you have to adjust the tilt trim frequently?
Q1b: Or, does a correct tilt trim angle mostly work for all situations?
If the engine needs to be trimmed for load, then power tilt-trim is a must.
If the hull is stable under varying loads then I would rather have the manual version, where a knowledgeable person can adjust trim angle by moving the metal rod after manually tilting the motor up.
The counter to "someone who can't lift the motor" is that they are also likely not to be running at speeds where trim will matter, nor will they likely know when the boat is trimmed out correctly. In other words, for a boater--young, old or lazy--who can't lift the manual tilt/trim, a power tilt/trim might get them in even more trouble than it is worth, that is, power trim solves problems that don't exist. I've seen this all over the whaler community: consider the number and price of 20-inch transom extension solutions for sale because folks can't be bothered to buy some epoxy, saw dust, and fiberglass
Q6: Would you pay an extra $1000 for powered tilt-trim?
Q7: How valuable is being able to adjust tilt-trim on a 13 while running?
Q8: most important of all: which propeller is best?
The 13 is really light weight at less than 600-lbs, but the hull shape so much drag.
If [readers] have a TOHATSU 25-HP EFI, I believe they have the 9.5 x 9.5 propeller, [suggested for boat weight of] 1,100 to 1, 700-lbs [and boat length of] 13 to 17-feet.
Part# 66: https://www.boats.net/product/tohatsu/3R0B645180?ref=1b384b92c7388502a5b3dced3a124f6e3042561e
The gear ratio is of the Mercury 25 FOURSTROKE is 1.92:1.
Here is the propeller suggestion table from Mercury
9"x12.5", 600-1000lbs, up to 16
9.25"x11", 900-1300lbs, 13' to 16
9.5"x9.5", 1100-1700lbs, 13' to 17]
9.75"x8", 1500lbs+, 16'+